The Lowdown on September’s Harvest Moon

The Full Moon of August 18, 2016 - the “Sturgeon Moon” - rising amid cloud over a wheatfield. This is a 5-exposure stack blended with luminosity masks, and shot with the Canon 60Da and 135mm telephoto.
The Full Moon of August 18, 2016 rises amid cloud over a wheat field. Friday night will see the rising of the annual Harvest Moon. Credit: Alan Dyer

It’s that wonderful time of year again when the Harvest Moon teeters on the horizon at sunset. You can watch the big orange globe rise on Friday (Sept. 16) from your home or favorite open vista just as soon as the Sun goes down. Despite being one of the most common sky events, a Full Moon rise still touches our hearts and minds every time. No matter how long I live, there will never be enough of them.

Friday night's Harvest Moon rises around sunset in the faint constellation Pisces the fish. Two fists above and left of the Moon, look for the four stars that outline the massive asterism of Pegasus the flying horse. Stellarium
Friday night’s Harvest Moon rises around sunset in the faint constellation Pisces the fish. Watch for it to come up almost due east around the time of sunset. Once the sky gets dark, look two fists above and left of the Moon for the four stars that outline the spacious asterism of Pegasus the flying horse. Stellarium

To see a moonrise, the most important information you need is the time the moon pops up for your city, which you’ll find by using this Moonrise and Moonset calculator. Once you know when our neighborly night light rises, pre-arrange a spot you can walk or drive to 10-15 minutes beforehand. The waiting is fun. Who will see it first? I’ll often expect to see the Moon at a certain point along the horizon then be surprised it’s over there.

A photographer finds just the right spot in Duluth along Lake Superior to photograph the Full Moon rise. The flattened shape of the Moon is caused by the layer of denser air closer to the horizon refracting or bending the bottom half of the Moon more strongly than the thinner air n
A photographer finds just the right spot in Duluth along Lake Superior to photograph a rising Full Moon. The flattened shape of the Moon is caused by the layer of denser air closer to the horizon refracting or bending the bottom half of the Moon more strongly than the thinner air along the top limb. In effect, refraction “lifts” the bottom half of the Moon upward into the top to give it a squashed appearance. Once the Moon rises high enough so we see it through much thinner (less dense) air, refraction becomes negligible and the Moon assumes its more familiar circular shape.  Credit: Bob King

Depending on how low to the horizon you can see, it’s possible, especially over water, to catch the first glimpse of lunar limb breaching the horizon. This still can be a tricky feat because the Moon is pale, and when it rises, shows little contrast against the still-bright sky. Since the Moon moves about one outstretched fist to the east (left in the northern hemisphere) each night, if you wait until one night after full phase, the Moon will rise in a much darker sky and appear in more dramatic contrast against the sky background.

As the Moon rises, we peer through hundreds of miles of the lower atmosphere, where the air is densest and dustiest. Aerosols scatter much of the blues and greens in moonlight away, leaving orange and red. Turbulence and varying air densities along the line of sight can create all manner of distortions of the lunar disk. This photo sequence showing an extraordinary moonset was taken from the shores of Garrison Lake in Port Orford, Oregon. The camera was facing west; looking across the lake, beyond the narrow foredune and out toward the Pacific Ocean. A very clear atmosphere enabled me to watch the Moon set all the way down to the horizon. The distortion that occurred as it descended was quite remarkable -- the Moon's shape was changing as fast as I could snap a picture.  Credit: Randy Scholten
This photo sequence showing an extraordinary moonset taken from the shores of Garrison Lake in Port Orford, Oregon. “The distortion that occurred as it descended was quite remarkable — the Moon’s shape was changing as fast as I could snap a picture,” said photographer Randy Scholten. As the Moon rises, we peer through hundreds of miles of the lower atmosphere, where the air is densest and dustiest. Aerosols scatter much of the blues and greens in moonlight away, leaving orange and red. Turbulence and varying air densities along the line of sight can create all manner of distortions of the lunar disk. Credit: Randy Scholten

Look closely at the rising Moon with both naked eye and binoculars and you might just see a bit of atmospheric sorcery at work. Refraction, illustrated the icy moonrise image above, is the big one. It creates the squashed Moon shape. But more subtle things are happening that depend on how turbulent or calm the air is along your line of sight to our satellite.

Clouds add their own beauty and mystery to the rising Moon. Credit: Bob King
Clouds add their own beauty and mystery to the rising Moon. Credit: Bob King

Rippling waves “sizzling” around the lunar circumference can be striking in binoculars though the effect is quite subtle with the naked eye. Much easier to see without any optical aid are the weird shapes the Moon can assume depending upon the state of the atmosphere. It can looked stretched out like a hot air balloon, choppy with a step-like outline around its bottom or top, square, split into two moons or even resemble a “mushroom cloud”.

If you make a point to watch moonrises regularly, you’ll become acquainted as much with Earth’s atmosphere as with the alien beauty of our sole satellite.

This Full Moon is special in at least two ways. First, it will undergo a penumbral eclipse for skywatchers across eastern Europe, Africa, Asia and Australia. Observers there should watch a dusky gray shading over the upper or northern half of the Moon around the time of maximum eclipse. The link will take you to Dave Dickinson’s excellent article that appeared earlier here at Universe Today.

The angle of the moon’s path to the horizon makes all the difference in moonrise times. At full phase in spring, the path tilts steeply southward, delaying successive moonrises by over an hour. In September, the moon’s path is nearly parallel to the horizon with successive moonrises just 20+ minutes apart. Times are shown for the Duluth, Minn. region. Illustration: Bob King
The angle of the moon’s path to the horizon makes all the difference in moonrise times. At full phase in spring, the path tilts steeply southward, delaying successive moonrises by over an hour. In September, the moon’s path is nearly parallel to the horizon with successive moonrises just 20+ minutes apart. Times shown are for illustration only  — so you can see the dramatic different in rise times — and don’t refer necessarily to Friday night’s moonrise. Illustration: Bob King

In the northern hemisphere, September’s Full Moon is named the Harvest Moon, defined as the Full Moon closest to the autumnal equinox, which occurs at 9:21 a.m. CDT (14:21 UT) on the 22nd. Normally, the Moon rises on average about 50 minutes later each night as it moves eastward along its orbit. But at Harvest Moon, successive moonrises are separated by a half-hour or less as viewed from mid-northern latitudes. The short gap of time between between bright risings gave farmers in the days before electricity extra light to harvest their crops, hence the name.

Use your imagination and you can see any of several figures in the Full Moon composed of contrasting maria and highlands.
Use your imagination and you can see any of several figures in the Full Moon composed of contrasting maria and highlands.

Why the faster-than-usual moonrises? Every September, the Full Moon’s nightly travels occur at a shallow angle to the horizon; as the moon scoots eastward, it’s also moving northward this time of year as shown in the illustration above. The northern and eastward motions combine to make the Moon’s path nearly level to the horizon. For several nights in a row, it only takes a half-hour for the Earth’s rotation to carry the Moon up from below the horizon. In spring, the angle is steep because the Moon is then moving quickly southward along or near the ecliptic, the path it takes around the sky.  Rising times can exceed an hour.

As you gaze at the Moon over the next several nights, take in the contrast between its ancient crust, called the lunar highlands, and the darker seas (also known as maria, pronounced MAH-ree-uh). The crust appears white because it’s rich in calcium and aluminum, while the maria are slightly more recent basaltic lava flows rich in iron, which lends them a darker tone. Thanks to these two different types of terrain it’s easy to picture a male or female face or rabbit or anything your imagination desires.

Happy moongazing!

August Full Moon Anticipates September’s Total Lunar Eclipse

Who doesn’t love a Full Moon? Occurring about once a month, they never wear out their welcome. Each one becomes a special event to anticipate. In the summer months, when the Moon rises through the sultry haze, atmosphere and aerosols scatter away so much blue light and green light from its disk, the Moon glows an enticing orange or red.

At Full Moon, we’re also more likely to notice how the denser atmosphere near the horizon squeezes the lunar disk into a crazy hamburger bun shape. It’s caused by atmospheric refraction.  Air closest to the horizon refracts more strongly than air near the top edge of the Moon, in effect “lifting” the bottom of the Moon up into the top. Squished light! We also get to see all the nearside maria or “seas” at full phase, while rayed craters like Tycho and Copernicus come into their full glory, looking for all the world like giant spatters of white paint even to the naked eye.

At full phase, the Moon lies directly opposite the Sun on the other side of Earth. Sunlight hits the Moon square on and fully illuminates the Earth-facing hemisphere. Credit: Bob King
At full phase, the Moon lies directly opposite the Sun on the other side of Earth. Sunlight hits the Moon square on and fully illuminates the Earth-facing hemisphere. Credit: Bob King

Tomorrow night (August 29), the Full Sturgeon Moon rises around sunset across the world. The name comes from the association Great Lakes Indian groups made between the August moon and the best time to catch sturgeon. Next month’s moon is the familiar Harvest Moon; the additional light it provided at this important time of year allowed farmers to harvest into the night.

A Full Moon lies opposite the Sun in the sky exactly like a planet at opposition. Earth is stuck directly between the two orbs. As we look to the west  to watch the Sun go down, the Moon creeps up at our back from the eastern horizon. Full Moon is the only time the Moon faces Sun directly – not off to one side or another – as seen from Earth, so the entire disk is illuminated.

The moon provides the perfect backdrop for watching birds migrate at night. Observers with spotting scopes and small telescopes can watch the show anytime the moon is at or near full. Photo illustration: Bob King
The moon provides the perfect backdrop for watching birds migrate at night. Although a small telescope is best, you might see an occasional bird in binoculars, too. Credit: Bob King

If you’re a moonrise watcher like I am, you’ll want to find a place where you can see all the way down to the eastern horizon tomorrow night. You’ll also need the time of moonrise for your city and a pair of binoculars. Sure, you can watch a moonrise without optical aid perfectly well, but you’ll miss all the cool distortions happening across the lunar disk from air turbulence. Birds have also begun their annual migration south. Don’t be surprised if your glass also shows an occasional winged silhouette zipping over those lunar seas.

Because the Moon's orbit is tilted 5.1 degrees with respect to Earth's, it normally passes above or below Earth's shadow with no eclipse. Only when the lineup is exact, does the Moon then pass directly behind Earth and into its shadow. Credit: Bob King
Because the Moon’s orbit is tilted 5.1° with respect to Earth’s, it normally passes above or below Earth’s shadow with no eclipse — either lunar or solar. Only when the lineup is exact, does the Moon pass directly behind Earth and into its shadow. Credit: Bob King

Next month’s Full Moon is very special. A few times a year, the alignment of Sun, Earth and Moon (in that order) is precise, and the Full Moon dives into Earth’s shadow in total eclipse. That will happen overnight Sunday night-Monday morning September 27-28. This will be the final in the current tetrad of four total lunar eclipses, each spaced about six months apart from the other. I think this one will be the best of the bunch. Why?

The totally eclipsed moon on April 15, 2014 from Duluth, Minn. This was the first in the series of four eclipses called a tetrad. Some refer to this lunar eclipse as a “Blood Moon” because it coincides with the Jewish Passover. Credit: Bob King
The totally eclipsed moon on April 15, 2014 from Duluth, Minn. This was the first in the series of four eclipses called a tetrad. September’s totally eclipsed Moon will appear similar. The coloring comes from sunlight grazing the edge of Earth’s atmosphere and refracted by it into the planet’s shadow. Credit: Bob King
  • Convenient evening viewing hours (CDT times given) for observers in the Americas. Partial eclipse begins at 8:07 p.m., totality lasts from 9:11 – 10:23 p.m. and partial eclipse ends at 11:27 p.m. Those times mean that for many regions, kids can stay up and watch.
  • The Moon passes more centrally through Earth’s shadow than during the last total eclipse. That means a longer totality and possibly more striking color contrasts.
  • September’s will be the last total eclipse visible in the Americas until January 31, 2018. Between now and then, there will be a total of four minor penumbral eclipses and one small partial. Slim pickings.
Diagram showing the details of the upcoming total lunar eclipse. The event begins when the Moon treads into Earth's outer shadow (penumbra) at 7:12 p.m. CDT. Partial phases start at 8:07 and totality at 9:11. Credit: NASA / Fred Espenak
Diagram showing the details of the upcoming total lunar eclipse. The event begins when the Moon treads into Earth’s outer shadow (penumbra) at 7:12 p.m. CDT. Partial phases start at 8:07 and totality at 9:11. Credit: NASA / Fred Espenak

Not only will the Americas enjoy a spectacle, but totality will also be visible from Europe, Africa and parts of Asia. For eastern hemisphere skywatchers, the event will occur during early morning hours of September 28. Universal or UT times for the eclipse are as follows: Partial phase begin at 1:07 a.m., totality from 2:11 – 3:23 a.m. with the end of partial phase at 4:27 a.m.

Eclipse visibility map. Credit: NASA / Fred Espenak
September 27-28, 2015 eclipse visibility map. Credit: NASA / Fred Espenak

We’ll have much more coverage on the upcoming eclipse in future articles here at Universe Today. I hope this brief look will serve to whet your appetite and help you anticipate what promises to be one of the best astronomical events of 2015.

The Resplendent Inflexibility of the Rainbow

Children often ask simple questions that make you wonder if you really understand your subject.  An young acquaintance of mine named Collin wondered why the colors of the rainbow were always in the same order — red, orange, yellow, green, blue, indigo, violet. Why don’t they get mixed up? 

The familiar sequence is captured in the famous Roy G. Biv acronym, which describes the sequence of rainbow colors beginning with red, which has the longest wavelength, and ending in violet, the shortest. Wavelength — the distance between two successive wave crests — and frequency, the number of waves of light that pass a given point every second, determine the color of light.

The familiar colors of the rainbow spectrum with wavelengths shown in nanometers. Credit: NASA
The familiar colors of the rainbow spectrum with wavelengths shown in nanometers. Credit: NASA

The cone cells in our retinas respond to wavelengths of light between 650 nanometers (red) to 400 (violet). A nanometer is equal to one-billionth of a meter. Considering that a human hair is 80,000-100,000 nanometers wide, visible light waves are tiny things indeed.

So why Roy G. Biv and not Rob G. Ivy? When light passes through a vacuum it does so in a straight line without deviation at its top speed of 186,000 miles a second (300,000 km/sec). At this speed, the fastest known in the universe as described in Einstein’s Special Theory of Relativity, light traveling from the computer screen to your eyes takes only about 1/1,000,000,000 of second. Damn fast.

But when we look beyond the screen to the big, wide universe, light seems to slow to a crawl, taking all of 4.4 hours just to reach Pluto and 25,000 years to fly by the black hole at the center of the Milky Way galaxy. Isn’t there something faster? Einstein would answer with an emphatic “No!”

A laser beam (left) shining through a glass of water demonstrates how many times light changes speed — from 186,222 miles per second (mps) in air to 124,275 mps through the glass. It speeds up again to 140,430 mps in water, slows down when passing through the other side of the glass and then speeds up again when leaving the glass for the air. Credit: Bob King
A laser beam (left) shining through a glass of water demonstrates how many times light changes speed — from 186,222 miles per second (mps) in air to 124,275 mps through the glass. It speeds up again to 140,430 mps in water, slows down when passing through the other side of the glass and then speeds up again when leaving the glass for the air. Credit: Bob King

One of light’s most interesting properties is that it changes speed depending on the medium through which it travels. While a beam’s velocity through the air is nearly the same as in a vacuum, “thicker” mediums slow it down considerably. One of the most familiar is water. When light crosses from air into water, say a raindrop, its speed drops to 140,430 miles a second (226,000 km/sec). Glass retards light rays to 124,275 miles/second, while the carbon atoms that make up diamond crunch its speed down to just 77,670 miles/second.

Why light slows down is a bit complicated but so interesting, let’s take a moment to describe the process. Light entering water immediately gets absorbed by atoms of oxygen and hydrogen, causing their electrons to vibrate momentarily before it’s re-emitting as light. Free again, the beam now travels on until it slams into more atoms, gets their electrons vibrating and gets reemitted again. And again. And again.

A ray of light refracted by a plastic block. Notice that the light bends twice - once when it enters (moving from air to plastic) and again when it exits (plastic to air).
A ray of light refracted by a plastic block. Notice that the light bends twice – once when it enters (moving from air to plastic) and again when it exits (plastic to air). The beam slows down on entering and then speeds up again when it exits.

Like an assembly line, the cycle of absorption and reemission continues until the ray exits the drop. Even though every photon (or wave – your choice) of light travels at the vacuum speed of light in the voids between atoms, the minute time delays during the absorption and reemission process add up to cause the net speed of the light beam to slow down. When it finally leaves the drop, it resumes its normal speed through the airy air.

Light rays get bent or refracted when they move from one medium to another. We've all seen the "broken pencil" effect when light travels from air into water.
Light rays get bent or refracted when they move from one medium to another. We’ve all seen the “broken pencil” effect when light travels from air into water.

Let’s return now to rainbows. When light passes from one medium to another and its speed drops, it also gets bent or refracted. Plop a pencil in a glass half filled with water and and you’ll see what I mean.

Up to this point, we’ve been talking about white light only, but as we all learned in elementary science, Sir Isaac Newton conducted experiments with prisms in the late 1600s and discovered that white light is comprised of all the colors of the rainbow. It’s no surprise that each of those colors travels at a slightly different speed through a water droplet. Red light interacts only weakly with the electrons of the atoms and is refracted and slowed the least. Shorter wavelength violet light interacts more strongly with the electrons and suffers a greater degree of refraction and slowdown.

Isaac Newton used a prism to separate light into its familiar array of colors. Like a prism, a raindrop refracts  incoming sunlight, spreading it into an arc of rainbow colors  with a radius of 42. Left: NASA image, right, public domain with annotations by the author
Isaac Newton used a prism to separate light into its familiar array of colors. Like a prism, a raindrop refracts incoming sunlight, spreading it into an arc of rainbow colors with a radius of 42. The colors spread out when light enter the drop and then spread out more when they leave and speed up. Left: NASA image, right, public domain with annotations by the author

Rainbows form when billions of water droplets act like miniature prisms and refract sunlight. Violet (the most refracted) shows up at the bottom or inner edge of the arc. Orange and yellow are refracted a bit less than violet and take up the middle of the rainbow. Red light, least affected by refraction, appears along the arc’s outer edge.

Rainbows are often double. The secondary bow results from light reflecting a second time inside the raindrop. When it emerges, the colors are reversed (red on the bottom instead of top), but the order of colors is preserved. Credit: Bob King
Rainbows are often double. The secondary bow results from light reflecting a second time inside the raindrop. When it emerges, the colors are reversed (red on the bottom instead of top), but the order of colors is preserved. Credit: Bob King

Because their speeds through water (and other media) are a set property of light, and since speed determines how much each is bent as they cross from air to water, they always fall in line as Roy G. Biv. Or the reverse order if the light beam reflects twice inside the raindrop before exiting, but the relation of color to color is always preserved. Nature doesn’t and can’t randomly mix up the scheme. As Scotty from Star Trek would say: “You can’t change the laws of physics!”

So to answer Collin’s original question, the colors of light always stay in the same order because each travels at a different speed when refracted at an angle through a raindrop or prism.

Light of different colors have both different wavelengths (distance between successive wave crests) and frequencies. In this diagram, red light has a longer wavelength and more "stretched out" waves  compared to purple light of higher frequency. Credit: NASA
Light of different colors have both different wavelengths (distance between successive wave crests) and frequencies. In this diagram, red light has a longer wavelength and more “stretched out” waves compared to purple light of higher frequency. Credit: NASA

Not only does light change its speed when it enters a new medium, its wavelength changes,  but its frequency remains the same. While wavelength may be a useful way to describe the colors of light in a single medium (air, for instance), it doesn’t work when light transitions from one medium to another. For that we rely on its frequency or how many waves of colored light pass a set point per second.

Higher frequency violet light crams in 790 trillion waves per second (cycles per second) vs. 390 trillion for red. Interestingly, the higher the frequency, the more energy a particular flavor of light carries, one reason why UV will give you a sunburn and red light won’t.

When a ray of sunlight enters a raindrop, the distance between each successive crest of the light wave decreases, shortening the beam’s wavelength. That might make you think that that its color must get “bluer” as it passes through a raindrop. It doesn’t because the frequency remains the same.

We measure frequency by dividing the number of wave crests passing a point per unit time. The extra time light takes to travel through the drop neatly cancels the shortening of wavelength caused by the ray’s drop in speed, preserving the beam’s frequency and thus color. Click HERE for a further explanation.


Why prisms/raindrops bend and separate light

Before we wrap up, there remains an unanswered question tickling in the back of our minds. Why does light bend in the first place when it shines through water or glass? Why not just go straight through? Well, light does pass straight through if it’s perpendicular to the medium. Only if it arrives at an angle from the side will it get bent. It’s similar to watching an incoming ocean wave bend around a cliff. For a nice visual explanation, I recommend the excellent, short video above.

Oh, and Collin, thanks for that question buddy!

What Makes Mars Sunsets Different from Earth’s?

Even robots can’t tear their eyes from a beautiful sunset. NASA’s Mars Curiosity rover pointed its high resolution mast camera at the setting Sun to capture this 4-image sequence on April 15 at the conclusion of the mission’s 956th Martian day. While it resembles an earthly sunset, closer inspection reveals alien oddities.

A day on Mars lasts 24 hours and 39 minutes, so sunrise and sunset follow nearly the same rhythm as they do on Earth. When we eventually establish a base there, astronauts should be able to adjust to the planet’s day-night rhythm with relative ease. Jet lag would be worse.

But sunsets and sunrises offer a different palette of colors than they would on Earth. For starters, the Sun only radiates the equivalent of a partly cloudy afternoon’s worth of light. That’s because Mars’ average distance from the Sun is 141.6 million miles or about half again Earth’s distance. Increased distance reduces the intensity of sunlight.

Not only that, but the solar disk shrinks from the familiar 0.5° across we see from Earth to 0.35° at Mars. Here on the home planet, your little finger extended at arm’s length would cover the equivalent of two Suns. On Mars it would be three!

Wide view of sunset over Gusev Crater taken by NASA's Spirit Rover in 2005. Both blue aureole and pink sky are seen. Because of the fine nature of Martian dust, it can scatter blue light coming from the Sun forward towards the observer. Credit: NASA/JPL-Caltech
Wide view of sunset over Gusev Crater taken by NASA’s Spirit Rover in 2005. Both blue aureole and pink sky are seen. Because of the fine nature of Martian dust, it can scatter blue light coming from the Sun forward towards the observer. Credit: NASA/JPL-Caltech

What about color? Dust and other fine particles in the atmosphere scatter the blues and greens from the setting or rising Sun to color it yellow, orange and red. When these tints are reflected off clouds, sunset colors are amplified and spread about the sky, making us reach for that camera phone to capture the glory.

Things are a little different on Mars. The ever-present fine dust in the Martian atmosphere absorbs blue light and scatters the warmer colors, coloring the sky well away from the Sun a familiar ruddy hue. At the same time, dust particles in the Sun’s direction scatter blue light forward to create a cool, blue aureole near the setting Sun. If you were standing on Mars, you’d only notice the blue glow when the Sun was near the horizon, the time when its light passes through the greatest depth of atmosphere and dust.

This was the first sunset observed in color by Curiosity. The color has been calibrated and white-balanced to remove camera artifacts. Mastcam sees color much the way the human eye does, although it's a little less sensitive to blue. The Sun's disk itself appears pink because all the cooler colors have been scattered away, similar to why the Sun on Earth appears orange or red when near the horizon. Notice the rocky ridge in the foreground. Credit: NASA/JPL-Caltech/MSSS/Texas A&M Univ.
This was the first sunset observed in color by Curiosity. The color has been calibrated and white-balanced to remove camera artifacts. Mastcam sees color much the way the human eye does, although it’s a little less sensitive to blue. The Sun’s disk itself appears pink because all the cooler colors have been scattered away, similar to why the Sun on Earth appears orange or red when near the horizon. Notice the individual rocks poking up from the ridge in the foreground. Credit: NASA/JPL-Caltech/MSSS/Texas A&M Univ.

On Earth, blue light from the Sun is scattered by air molecules and spreads around the sky to create a blue canopy. Mars has less the 1% of Earth’s atmosphere, so we only notice the blue when looking through the greatest thickness of the Martian air (and dust) around the time of sunset and sunrise.


Sunset on Mars photographed by the Opportunity Rover released earlier this year

The video above of the setting Sun was made using stills taken by Opportunity, NASA’s “other” rover that’s been trekking across the Martian landscape for more than 10 years now. You can see a bit of pink in the Sun just before it sets as in the Curiosity photos, but there’s something else going on, too. Or not going on.

Sunrise of Lake Superior. Atmospheric refraction - bending of the Sun's light - flattens the disk into an oval shape. Credit: Lyle Anderson
Sunrise of Lake Superior. Atmospheric refraction – bending of the Sun’s light – flattens the disk into an oval shape. Credit: Lyle Anderson

When the Sun sets or rises on Earth, it’s squashed like a melon due to atmospheric refraction. Much thicker air adjacent to the horizon bends the Sun’s light upward, pushing the bottom of the solar disk into the top half which is less affected by refraction because it’s slightly higher. Once the Sun rises high enough, so we’re looking at it through less atmosphere, refraction diminishes and it becomes a circle again.

I’ve looked at both the Opportunity sunset and Curiosity sunset videos many times, and as far as I can tell, the Sun’s shape doesn’t change. At least it’s not noticeable to the casual eye. I bet you can guess why — the air is too thin to for refraction to make much of a difference.

Twilights linger longer on the Red Planet as well because dust lofted high into the stratosphere by storms continues to reflect the Sun’s light for two hours or more after sundown.

So you can see that sunset phenomena on Mars are different from ours because of the unique qualities of its atmosphere. I trust someone alive today will be the first human to see and photograph a Martian sunset. Hope I’m still around when that awesome pic pops up on Twitter.

Turning Stars Into Art

We all have cameras, and the sky’s an easy target, so why not have a little fun? Ever since I got my first camera at age 12 I wanted to shoot time exposures of the night sky. That and a tripod are all you need. Presented here for your enjoyment are a few oddball and yet oddly informative images of stars and planets.  Take the word “art” loosely! 

This is the pair to the Sirius image and shows Jupiter through the telescope. Notice how blandly white it appears. That's because Jupiter's disk is large enough to not  show twinkling (and color changes) caused by atmospheric turbulence as in the case of point-like Sirius.
Colorless mess. This is the companion to the Sirius image and shows Jupiter through the telescope. Notice how blandly white it appears. That’s because Jupiter’s disk is large enough to not show twinkling (and color changes) caused by atmospheric turbulence as in the case of point-like Sirius. Credit: Bob King
Orion's Belt and Sword trail in this time exposure made with a 200mm lens. The nearly perfectly parallel because the stars lie very near the celestial equator and were on the meridian at the time.
Pleasing parallels. Orion’s Belt and Sword trail in this time exposure made with a 200mm lens. The fuzzy pink streak is the Orion Nebula. They’re trails are nearly parallel because the stars all lie close to the celestial equator and were crossing the meridian at the time. Credit: Bob King
Star Trek Effect. OK, this was crazy to shoot. I centered Jupiter in the viewfinder, pressed the shutter button for a 20-second time exposure and slowly zoomed out from 70mm to 200mm on the telephoto lens. It took a few tries, because I was shooting blind, but even the rejects weren't too bad. Credit: Bob King
Star Trek Effect.  I centered Jupiter in the viewfinder, pressed the shutter button for a 20-second time exposure and slowly hand-zoomed the lens from 70mm to 200mm. It took a few tries because I was shooting blind, but even the rejects weren’t too bad. Credit: Bob King
Color by fog. The colors of stars are accentuated when photographed through fog or light cloud. Orion at right with the crescent moon at lower left. Credit: Bob King
Color by Fog. The colors of stars are accentuated when spread into a glowing disk by fog or light cloud. Orion  is at right with the crescent moon at lower left. Credit: Bob King
Snow flies.  During a time exposure taken on a snowy but partly cloudy night, snowflakes, illuminated by a yard light, streak about  beneath a Full Moon earlier this winter. Credit: Bob King
Snow flies. During a time exposure taken on a snowy but partly cloudy night, snowflakes, illuminated by a yard light, streak about beneath a Full Moon earlier this winter. Credit: Bob King
Stuttering Stars. For this image of the Big Dipper the camera was on a tracking mount. I left the shutter open for about a half hour, then covered the lens with a black cloth for a few minutes. After the cloth was removed, I started tracking and exposed the Dipper for a few minutes. During part of the exposure I used a diffusion filter in front of the lens to soften and enlarge the brightest stars. Credit: Bob King
Stuttering Stars. For this image of the Big Dipper the camera was on a tracking mount. I left the shutter open for about 25 minutes with the tracking turned off so the stars would trail.  Then the lens was covered with a black cloth for a few minutes to create a gap between this exposure and the next. After the cloth was removed, I started the tracking motor and kept the exposure running for a few minutes. A diffusion filter was used in front of the lens to soften and enlarge the brightest stars. Credit: Bob King

Guide to Tonight’s Big Harvest Moon

Tonight, September 8, the Harvest Moon rises the color of a fall leaf and spills its light across deserts, forests, oceans and cities. The next night it rises only a half hour later. And the next, too. The short gap of time between successive moonrises gave farmers in the days before electricity extra light to harvest their crops, hence the name.

The Harvest Moon is the full moon that falls closest to the autumnal equinox, the beginning of northern autumn. As the moon orbits the Earth, it moves eastward about one fist held at arm’s length each night and rises about 50 minutes later. You can see its orbital travels for yourself by comparing the moon’s nightly position to a bright star or constellation. 

This full Moon is also a Proxigean or Perigee Full “Supermoon” (find out more about that here), which means the Moon is in a spot in its elliptical orbit where it is closer to Earth near the time it is full, making it look up to 15% larger than average full Moon.

Around the time of Harvest Moon, the full moon's path is tilted at a shallow angle to the eastern horizon making with successive moonrises only about a half hour apart instead of the usual 50 minutes. Source: Stellarium
Around the time of Harvest Moon, the full moon’s path is tilted at a shallow angle to the eastern horizon making with successive moonrises only about a half hour apart instead of the usual 50 minutes. Source: Stellarium

50 minutes is the usual gap between moonrises. But it can vary from 25 minutes to more than an hour depending upon the angle the moon’s path makes to the eastern horizon at rise time. In September that path runs above the horizon at a shallow angle. As the moon scoots eastward, it’s also moving northward this time of year.

This northward motion isn’t as obvious unless you watch the moon over the coming week. Then you’ll see it climb to the very top of its monthly path when it’s high overhead at dawn. The northward motion compensates for the eastward motion, keeping the September full moon’s path roughly parallel to the horizon with successive rise times only ~30 minutes apart.

The angle of the moon’s path to the horizon makes all the difference in moonrise times. At full phase in spring, the path tilts steeply southward, delaying successive moonrises by over an hour. In September, the moon’s path is nearly parallel to the horizon with successive moonrises just 20+ minutes apart. Times are shown for the Duluth, Minn. region. Illustration: Bob King
The angle of the moon’s path to the horizon makes all the difference in moonrise times. At full phase in spring, the path tilts steeply southward, delaying successive moonrises by over an hour. In September, the moon’s path is nearly parallel to the horizon with successive moonrises just 30+ minutes apart. Times are shown for the Duluth, Minn. region. Illustration: Bob King

Exactly the opposite happened 6 months earlier this spring, when the moon’s path met the horizon at a steep angle. While it traveled the identical distance each night then as now, its tilted path dunked it much farther below the horizon night to night. The spring full moon moves east and south towards its lowest point in the sky. Seen from the northern hemisphere, that southward travel adds in extra time for the moon to reach the horizon and rise each successive night.

If all this is a bit mind-bending, don’t sweat it. Click HERE to find when the moon rises for your town and find a spot with a great view of the eastern horizon. You’ll notice the moon is orange or red at moonrise because the many miles of thicker atmosphere you look through when you gaze along the horizon scatters the shorter bluer colors from moonlight, tinting it red just as it does the sun.

A series of photos of the full moon setting over Earth's limb taken by from space by NASA astronaut Don Pettit on April 16, 2003. Refraction causes a celestial object's light to be bent upwards, so it appears higher than it actually is. The bottom half of the moon, closer to the horizon, is refracted strongest and "pushed" upward into the top half, making it look squished. Credit: NASA
A series of photos of the full moon setting over Earth’s limb taken by from space by NASA astronaut Don Pettit on April 16, 2003. Refraction causes a celestial object’s light to be bent upwards, so it appears higher than it actually is. The bottom half of the moon, closer to the horizon, is refracted strongest and “pushed” upward into the top half, making it look squished. Credit: NASA

The moon will also appear squished due to atmospheric refraction. Air is densest right at the horizon and refracts or bends light more strongly than the air immediately above it. Air “lifts” the bottom of the moon – which is closer to the horizon – more than the top, squishing the two halves together into an egg or oval shape.

How we perceive the moon's size may have much to do with what's around it. In this illustration, most of us seen the bottom moon as smaller, but they're both exactly the same size. Crazy, isn't it? Credit: NASA
How we perceive the moon’s size may have much to do with what’s around it. In this illustration, most of us seen the bottom moon as smaller, but they’re both exactly the same size. Crazy, isn’t it? Credit: NASA

You may also be entranced Monday night by the Moon Illusion, where the full moon appears unnaturally large when near the horizon compared to when viewed higher up. No one has come up with a complete explanation for this intriguing aspect of our perception, but the link above offers some interesting hypotheses.

Can you see craters with your naked eye? Yes! Try tonight through Wednesday night. Plato is the trickiest. Credit: Bob King
Can you see craters with your naked eye? Yes! Try tonight through Wednesday night. Plato is the trickiest. Credit: Bob King

Finally, full moon is an ideal time to see several lunar craters with the naked eye. They’re not the biggest, but all, except Plato, are surrounded by bright rays of secondary impact craters that expand their size and provide good contrast against the darker lunar “seas”. Try with your eyes alone first, and if you have difficulty, use binoculars to get familiar with the landscape and then try again with your unaided eyes.

In contrast to the other craters, Plato is dark against a bright landscape. It’s a true challenge – I’ve tried for years but still haven’t convinced myself of seeing it. The others are easier than you’d think. Good luck and clear skies!

If you don’t have clear skies, Slooh will broadcast the “Super Harvest Moon” live from the Institute of Astrophysics of the Canary Islands, off the coast of Africa. Slooh’s live coverage will begin at 6:30 PM PDT / 9:30 PM EDT /01:30 UTC (8/9) – International times here. Slooh hosts are Geoff Fox and Slooh astronomer Bob Berman. Viewers can ask questions during the show by using hashtag #Sloohsupermoon. Watch below:

Astronomy Cast Ep. 254: Reflection and Refraction

Light can do some pretty strange stuff, like pass through objects and bounce off them; it can be broken up and recombined. In fact, everything we “see” is actually the end result of reflection and refraction of light. Time to understand how it all works.

Remember that we record every episode of Astronomy Cast as a live Google+ Hangout on Mondays at 12 pm PST / 3 pm EST / 2000 GMT. You can watch us record the episode and even jump into the Hangout and ask us some questions. Follow Fraser on Google+ to see when it happens.

Dispersion of Light

Dispersion of Light

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Look up into the rainy sky! What do you see? Well, if its just rained and the sun is once again shining, chances are you see a rainbow. Always a lovely sight isn’t it? But why is it that after a rainstorm, the air seems to catch the light in just the right way to produce this magnificent natural phenomenon? Much like stars, galaxies, and the flight of a bumblebee, some complicated physics underlie this beautiful act of nature. For starters, this effect, where light is broken into the visible spectrum of colors, is known as the Dispersion of Light. Another name for it is the prismatic effect, since the effect is the same as if one looked at light through a prism.

To put it simply, light is transmitted on several different frequencies or wavelengths. What we know as “color” is in reality the visible wavelengths of light, all of which travel at different speeds through different media. In other words, light moves at different speed through the vacuum of space than it does through air, water, glass or crystal. And when it comes into contact with a different medium, the different color wavelengths are refracted at different angles. Those frequencies which travel faster are refracted at a lower angle while those that travel slower are refracted at a sharper angle. In other words, they are dispersed based on their frequency and wavelength, as well as the materials Index of Refraction (i.e. how sharply it refracts light).

The overall effect of this – different frequencies of light being refracted at different angles as they pass through a medium – is that they appear as a spectrum of color to the naked eye. In the case of the rainbow, this occurs as a result of light passing through air that is saturated with water. Sunlight is often referred to as “white light” since it is a combination of all the visible colors. However, when the light strikes the water molecules, which have a stronger index of refraction than air, it disperses into the visible spectrum, thus creating the illusion of a colored arc in the sky.

Now consider a window pane and a prism. When light passes through glass that has parallel sides, the light will return in the same direction that it entered the material. But if the material is shaped like a prism, the angles for each color will be exaggerated, and the colors will be displayed as a spectrum of light. Red, since it has the longest wavelength (700 nanometers) appears at the top of the spectrum, being refracted the least. It is followed shortly thereafter by Orange, Yellow, Green, Blue, Indigo and Violet (or ROY G. GIV, as some like to say). These colors, it should be noted, do not appear as perfectly distinct, but blend at the edges. It is only through ongoing experimentation and measurement that scientists were able to determine the distinct colors and their particular frequencies/wavelengths.

We have written many articles about dispersion of light for Universe Today. Here’s an article about the refractor telescope, and here’s an article about visible light.

If you’d like more info on the dispersion of light, check out these articles:
dispersion of Light by Prisms
Q & A: Dispersion of Light

We’ve also recorded an episode of Astronomy Cast all about the Hubble Space Telescope. Listen here, Episode 88: The Hubble Space Telescope.

Sources:
http://en.wikipedia.org/wiki/Refractive_index
http://en.wikipedia.org/wiki/Dispersion_%28optics%29
http://www.physicsclassroom.com/class/refrn/u14l4a.cfm
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=415.0
http://www.school-for-champions.com/science/light_dispersion.htm