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Can you feel the heat? If you find yourself north of the equator, astronomical summer kicks off today with the arrival of the summer solstice. In the southern hemisphere, the reverse is true, as today’s solstice marks the start of winter.
Thank our wacky seasons, and the 23.4 degree tilt of the Earth’s axis for the variation in insolation. Today, all along the Tropic of Cancer at latitude 23.4 degrees north, folks will experience what’s known as Lahiana Noon, as the Sun passes through the zenith directly overhead. Eratosthenes first noted this phenomena in 3rd century BC from an account in the town of Syene (modern day Aswan), 925 kilometers to the south of Alexandria, Egypt. The account mentioned how, at noon on the day of the solstice, the Sun shined straight down a local well, and cast no shadows. He went on to correctly deduce that the differing shadow angles between the two locales is due to the curvature of the Earth, and went on to calculate the curvature of the planet for good measure. Not a bad bit of reasoning, for an experiment that you can do today.
And although we call it the Tropic of Cancer, and the astrological sign of the Crab begins today as the Sun passes 90 degrees longitude along the ecliptic plane as seen from Earth, the Sun now actually sits in the astronomical constellation of Taurus on the June northward solstice. Thank precession; live out a normal 72 year human life span, and the solstice will move one degree along the ecliptic—stick around about 26,000 years, and it will complete one circuit of the zodiac. That’s something that your astrologer won’t tell you.
The solstice in the early 21st century actually falls on June 20th, thanks to the ‘reset’ the Gregorian calendar received in 2000 from the addition of a century year leap day. The actual moment the Sun reaches its northernmost declination today and slowly reverses its apparent motion is 22:34 Universal Time (UT). In 2016, the Moon reaches Full just 11 hours to the solstice. The last time a Full Moon fell within 24 hours of a solstice was December 2010, and we had a total lunar eclipse to boot. Such a coincidence won’t occur again until December 2018. You get a good study in celestial mechanics 101 tonight, as the Full Moon rises opposite to the setting Sun. The Moon occupies the southern region of the sky where the Sun will reside this December during the other solstice, when the Full Moon will then ride high in the night sky, and gets ever higher as we head towards a Major Lunar Standstill in 2025.
Of course, this motion of the Sun through the year is all an illusion from our terrestrial biased viewpoint. We’re actually racing around the Sun to the tune of 30 kilometers per second. You wouldn’t know it as summer heats up in the northern hemisphere, but we’re headed towards aphelion or the farthest point from the Sun for the Earth on July 4th at 152 million kilometers or 1.017 astronomical units (AU) distant. And the latest sunset as seen from latitude 40 degrees actually occurs on June 27th at 7:33 PM (not accounting for Daylight Saving Time) go much further north (like the Canadian Maritimes or the UK) and true astronomical darkness never occurs in late June.
And speaking of the Sun, we’re wrapping up the end of the 11 year solar cycle this year… and there are hints that we may be in for another profound solar minimum similar to 2009. We’ve already had a brief spotless stretch last month, and some solar astronomers have predicted that solar cycle #25 may be absent all together. This means a subsidence in aurorae, and an uncharacteristically blank Sol.
But don’t despair and pack it in for the summer. As a consolation prize, high northern latitudes have in recent years played host to electric blue noctilucent clouds near the June solstice. Also, the International Space Station enters a second period of full illumination through the entire length of its orbit from July 26th to 28th, making for the possibility of seeing multiple passes in a single night.
And folks in the Islamic world (and travelers such as ourselves currently in Morocco) can rejoice, as the Full Moon means that we’re half way through the fasting lunar month Ramadan. This is an especially tough one, as Ramadan 2016 goes right through the summer solstice, making for only a brief six hour span to break the fast each night and prepare for another 18 hour long stretch… and to repeat this pattern for 29 days straight. It’s a fascinating time of night markets and celebration, but for travelers, it also means odd opening hours and delays.
See any curious solstice shadow alignments in your neighborhood today?
Happy Lahiana Noon… from here on out, northern viewers slowly start to take back the night!
So, have you been following the path of the waning Moon through the dawn sky this week? The slender Moon visits some interesting environs over the coming weekend, and heralds the start of Ramadan across the Islamic world next week.
First up, the planet Mercury rises an hour before the Sun in the dawn this week. Mercury reaches greatest elongation west of the Sun on Sunday, June 5th at 9:00 Universal Time (UT).
The slender waning crescent Moon passes less than one degree from +0.8 magnitude Mercury (both 24 degrees from the Sun) on the morning of Friday, June 3rd at 10:00 UT. While this is a close shave worldwide, the Moon will actually occult (pass in front of) Mercury for a very few observers fortunate enough to be based on the Falkland Islands in the southern Atlantic.
The Moon is 5.2% illuminated and 41 hours from New during the occultation. Meanwhile, Mercury shines at magnitude +0.8 and displays an 8.6” 33.5% illuminated disk during the event. Also, watch for ashen light or Earthshine faintly lighting up the nighttime side of the Moon. You’re seeing sunlight, bounced off of the land, sea and (mostly) cloud tops of the fat waxing gibbous Earth back on to the lunar surface, one light-second away. The Big Bear Solar Observatory has a project known as Project Earthshine which seeks to measure and understand the changes in albedo (known as global dimming) and its effects on climate change.
The Moon occults Mercury three times in 2016. Occultations of the innermost planet are especially elusive, as they nearly always occur close to the Sun under a daytime sky. This week’s occultation occurs less than 48 hours from greatest elongation; the last time one was closer time-wise was March 5th, 2008, and this won’t be topped until February 18th, 2026, with an occultation of Mercury by the Moon just 18 hours prior to greatest elongation. And speaking of which, can you spy +0.8 magnitude Mercury near the crescent Moon on Friday… during the daytime? Use binocs, note where Mercury was in relation to the Moon before sunrise, but be sure to physically block that blinding Sun behind a building or hill!
The Moon also passes less than five degrees from the planet Venus on June 5th at 2:00 UT, though both are only 2 degrees from the Sun. Fun fact: the bulk of the Sun actually occults Venus for 47 hours as seen from the Earth from June 6th through June 8th.
You can observe the passage of Venus through the 15 degree wide field of view of SOHO’s LASCO C3 camera over the next few weeks until July 5th.
Venus reaches superior conjunction on the far side of the Sun 1.74 astronomical units (AU) from the Earth at 21:00 UT on Monday, June 6th.
New Moon occurs at 4:00 UT on Sunday, June 5th, marking the start of lunation 1156.
The Moon and Ramadan
The first sighting of the slim crescent Moon also marks the start of the month of Ramadan (Ramazan in Turkey) on the Islamic calendar. Unlike the western Gregorian calendar, which is strictly solar-based, and the Jewish calendar, which seeks to reconcile lunar and solar cycles, the Islamic is solely based on the 29.5 synodic period of the Moon. This means that it moves forward on average 11 days per Gregorian year. The hallmark of Ramadan is fasting from dawn to dusk, and Ramadan 2016 is an especially harsh one, falling across the northern hemisphere summer solstice (and the longest day of the year) on June 20th. The earliest sunrise occurs on June 14th, and latest sunset on June 27th for latitude 40 degrees north. Finally, the Earth reaches aphelion or its farthest point from the Sun on July 4th at 1.01675 AU or 157.5 million kilometers distant.
In 2016, the Moon will first likely be spotted from the west coast of South America on Sunday night June 5th, though many locales worldwide may not see the Moon until June 6th. There can be some disparity as to just when Ramadan starts based on the first sighting of the crescent Moon. The Islamic calendar is also unique in that it still relies on direct observation of the waxing crescent Moon. Other calendars often use an estimated approximation in a bid to keep their timekeeping in sync with the heavens. The computus estimation (not a supervillain, though it certainly sounds like one!) used by the Catholic Church to predict the future date of Easter, for example, fixes the vernal equinox on March 21st, though it actually falls on March 20th until 2048, when it actually slips to March 19th.
Ramadan has been observed on occasion in space by Muslim astronauts, and NASA even has guidelines stipulating that observant astros will follow the same protocols as their departure point from the Earth (in the foreseeable future, that’s the Baikonur Cosmodrome in Kazakhstan.
Can you see the open cluster M35, just six degrees north (right) of the thin crescent Moon on the evening of Monday, June 6th?
We think its great to see direct astronomical observation still having a hand in everyday human affairs. This also holds a special significance to us, as we’re currently traveling in Morocco.
Don’t miss the meeting of Mercury and the Moon on Friday morning, and the return of the Moon to the dusk skies next week.
For most of here on planet Earth, sunrise, sunset, and the cycle of day and night (aka. the diurnal cycle) are just simple facts of life. As a result of seasonal changes that happen with every passing year, the length of day and night can vary – and be either longer or shorter – by just a few hours. But in some regions of the world (i.e. the poles) the Sun does not set during certain times of the year. And there are also seasonal periods where a single night can last many days.
Naturally, this gives rise to certain questions. Namely, what causes the cycle of day and night, and why don’t all places on the planet experience the same patterns? As with many other seasonal experiences, the answer has to do with two facts: One, the Earth rotates on its axis as it orbits the Sun. And two, the fact that Earth’s axis is tilted.
Earth’s rotation occurs from west to east, which is why the Sun always appears to be rising on the eastern horizon and setting on the western. If you could view the Earth from above, looking down at the northern polar region, the planet would appear to be rotating counter-clockwise. However, viewed from the southern polar region, it appears to be rotating clockwise.
The Earth rotates once in about 24 hours with respect to the Sun and once every 23 hours 56 minutes and 4 seconds with respect to the stars. What’s more, its central axis is aligned with two stars. The northern axis points outward to Polaris, hence why it is called “the North Star”, while its southern axis points to Sigma Octantis.
As already noted, due to the Earth’s axial tilt (or obliquity), day and night are not evenly divided. If the Earth’s axis were perpendicular to its orbital plane around the Sun, all places on Earth would experience equal amounts of day and night (i.e. 12 hours of day and night, respectively) every day during the year and there would be no seasonal variability.
Instead, at any given time of the year, one hemisphere is pointed slightly more towards the Sun, leaving the other pointed away. During this time, one hemisphere will be experiencing warmer temperatures and longer days while the other will experience colder temperatures and longer nights.
Of course, since the Earth is rotating around the Sun and not just on its axis, this process is reversed during the course of a year. Every six months, the Earth undergoes a half orbit and changes positions to the other side of the Sun, allowing the other hemisphere to experience longer days and warmer temperatures.
Consequently, in extreme places like the North and South pole, daylight or nighttime can last for days. Those times of the year when the northern and southern hemispheres experience their longest days and nights are called solstices, which occur twice a year for the northern and southern hemispheres.
The Summer Solstice takes place between June 20th and 22nd in the northern hemisphere and between December 20th and 23rd each year in the southern hemisphere. The Winter Solstice occurs at the same time but in reverse – between Dec. 20th and 23rd for the northern hemisphere and June 20th and 22nd for the southern hemisphere.
According to NOAA, around the Winter Solstice at the North Pole there will be no sunlight or even twilight beginning in early October, and the darkness lasts until the beginning of dawn in early March. Conversely, around the Summer Solstice, the North Pole stays in full sunlight all day long throughout the entire summer (unless there are clouds). After the Summer Solstice, the sun starts to sink towards the horizon.
Another common feature in the cycle of day and night is the visibility of the Moon, the stars, and other celestial bodies. Technically, we don’t always see the Moon at night. On certain days, when the Moon is well-positioned between the Earth and the Sun, it is visible during the daytime. However, the stars and other planets of our Solar System are only visible at night after the Sun has fully set.
The reason for this is because the light of these objects is too faint to be seen during daylight hours. The Sun, being the closest star to us and the most radiant object visible from Earth, naturally obscures them when it is overhead. However, with the Earth tilted away from the Sun, we are able to see the Moon radiating the Sun’s light more clearly, and the stars light is detectable.
On an especially clear night, and assuming light pollution is not a major factor, the glowing band of the Milky Way and other clouds of dust and gas may also be visible in the night sky. These objects are more distant than the stars in our vicinity of the Galaxy, and therefore have less luminosity and are more difficult to see.
Another interesting thing about the cycle of day and night is that it is getting slower with time. This is due to the tidal effects the Moon has on Earth’s rotation, which is making days longer (but only marginally). According to atomic clocks around the world, the modern day is about 1.7 milliseconds longer than it was a century ago – a change which may require the addition of more leap seconds in the future.
Ever since the 16th century when Nicolaus Copernicus demonstrated that the Earth revolved around in the Sun, scientists have worked tirelessly to understand the relationship in mathematical terms. If this bright celestial body – upon which depends the seasons, the diurnal cycle, and all life on Earth – does not revolve around us, then what exactly is the nature of our orbit around it?
For several centuries, astronomers have applied the scientific method to answer this question, and have determined that the Earth’s orbit around the Sun has many fascinating characteristics. And what they have found has helped us to understanding why we measure time the way we do.
First of all, the speed of the Earth’s orbit around the Sun is 108,000 km/h, which means that our planet travels 940 million km during a single orbit. The Earth completes one orbit every 365.242199 mean solar days, a fact which goes a long way towards explaining why need an extra calendar day every four years (aka. during a leap year).
The planet’s distance from the Sun varies as it orbits. In fact, the Earth is never the same distance from the Sun from day to day. When the Earth is closest to the Sun, it is said to be at perihelion. This occurs around January 3rd each year, when the Earth is at a distance of about 147,098,074 km.
The average distance of the Earth from the Sun is about 149.6 million km, which is also referred to as one astronomical unit (AU). When it is at its farthest distance from the Sun, Earth is said to be at aphelion – which happens around July 4th where the Earth reaches a distance of about 152,097,701 km.
And those of you in the northern hemisphere will notice that “warm” or “cold” weather does not coincide with how close the Earth is to the Sun. That is determined by axial tilt (see below).
Next, there is the nature of the Earth’s orbit. Rather than being a perfect circle, the Earth moves around the Sun in an extended circular or oval pattern. This is what is known as an “elliptical” orbit. This orbital pattern was first described by Johannes Kepler, a German mathematician and astronomer, in his seminal work Astronomia nova (New Astronomy).
After measuring the orbits of the Earth and Mars, he noticed that at times, the orbits of both planets appeared to be speeding up or slowing down. This coincided directly with the planets’ aphelion and perihelion, meaning that the planets’ distance from the Sun bore a direct relationship to the speed of their orbits. It also meant that both Earth and Mars did not orbit the Sun in perfectly circular patterns.
In describing the nature of elliptical orbits, scientists use a factor known as “eccentricity”, which is expressed in the form of a number between zero and one. If a planet’s eccentricity is close to zero, then the ellipse is nearly a circle. If it is close to one, the ellipse is long and slender.
Earth’s orbit has an eccentricity of less than 0.02, which means that it is very close to being circular. That is why the difference between the Earth’s distance from the Sun at perihelion and aphelion is very little – less than 5 million km.
Third, there is the role Earth’s orbit plays in the seasons, which we referred to above. The four seasons are determined by the fact that the Earth is tilted 23.4° on its vertical axis, which is referred to as “axial tilt.” This quirk in our orbit determines the solstices – the point in the orbit of maximum axial tilt toward or away from the Sun – and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular.
In short, when the northern hemisphere is tilted away from the Sun, it experiences winter while the southern hemisphere experiences summer. Six months later, when the northern hemisphere is tilted towards the Sun, the seasonal order is reversed.
In the northern hemisphere, winter solstice occurs around December 21st, summer solstice is near June 21st, spring equinox is around March 20th and autumnal equinox is about September 23rd. The axial tilt in the southern hemisphere is exactly the opposite of the direction in the northern hemisphere. Thus the seasonal effects in the south are reversed.
While it is true that Earth does have a perihelion, or point at which it is closest to the sun, and an aphelion, its farthest point from the Sun, the difference between these distances is too minimal to have any significant impact on the Earth’s seasons and climate.
Another interesting characteristic of the Earth’s orbit around the Sun has to do with Lagrange Points. These are the five positions in Earth’s orbital configuration around the Sun where where the combined gravitational pull of the Earth and the Sun provides precisely the centripetal force required to orbit with them.
The five Lagrange Points between the Earth are labelled (somewhat unimaginatively) L1 to L5. L1, L2, and L3 sit along a straight line that goes through the Earth and Sun. L1 sits between them, L3 is on the opposite side of the Sun from the Earth, and L2 is on the opposite side of the Earth from L1. These three Lagrange points are unstable, which means that a satellite placed at any one of them will move off course if disturbed in the slightest.
The L4 and L5 points lie at the tips of the two equilateral triangles where the Sun and Earth constitute the two lower points. These points liem along along Earth’s orbit, with L4 60° behind it and L5 60° ahead. These two Lagrange Points are stable, hence why they are popular destinations for satellites and space telescopes.
The study of Earth’s orbit around the Sun has taught scientists much about other planets as well. Knowing where a planet sits in relation to its parent star, its orbital period, its axial tilt, and a host of other factors are all central to determining whether or not life may exist on one, and whether or not human beings could one day live there.
This beautiful image from the Solar Dynamics Observatory provides a view of our Sun on the solstice (winter solstice for the northern hemisphere and summer solstice for the southern hemisphere,) at 11:12 UTC, December 21, 2012. No killer solar flares, no apocalypse, just pure beauty and the life-giving light and warmth from Old Sol.
June 21st, 2011 is Summer Solstice – the longest day of the year.
This is the time when the Sun is at its highest or most northerly point in the sky in the Northern Hemisphere and when we receive the most hours of daylight. If you live in the Southern Hemisphere it is the reverse, so you will be having “Winter Solstice.”
Also known as “Midsummer” the Summer Solstice gets its name from the Latin for sol (sun) and sistere (to stand still). The Sun reaches its most Northerly point and momentarily stands still before starting its journey South in the sky again until it reaches its most Southerly point “Winter Solstice”, before repeating the cycle. This is basically how we get our seasons.
It’s not actually the Sun that moves North or South over the seasons although it may appear so. It’s the Earths axial tilt that causes the Sun to change position in the sky as the Earth orbits the Sun throughout the year.
Summer Solstice/ Midsummer is steeped in ancient folklore especially in Northern Europe with the most famous place directly related to it being Stonehenge, where the sun has been worshiped for thousands of years.
The Sun reaches its most Northerly point in the sky at 17:16 UTC momentarily and from that point forward starts to make its way South. This means the days will get shorter and shorter until Winter Solstice in December.