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A lunar day is the length of time it takes for the Moon to make one complete rotation on its axis compared to the Sun. This is important because the Moon is tidally locked with respect to the Earth. So it always points the same face towards the Earth as it goes around the planet. So, how long is a day on the Moon?
The lunar day lasts 29 days, 12 hours and 44 minutes. And this the same time it takes for the Moon to orbit around the Earth.
With respect to the background stars, however, the Moon only takes 27 days and 7 hours for the sky to completely rotate back to its original position.
So why is there a difference?
As the Earth and Moon are orbiting around the Sun, they complete a circle over the course of the year. Each time the Moon goes around the Earth, it needs to go a little further to get the Sun back into the same position.
If you ever get the opportunity to stand on the surface of the Moon, and look at the Earth, our planet would always remain in the exact same position in the sky. The Sun, on the other hand, will still rise, move across the sky and then set. Of course, an average day will last 29 days, 12 hours and 44 minutes until the Sun returns to the same position in the sky.
Astronomers say that the Moon is tidally locked to the Earth. At some point in the distant past, the Moon rotated more rapidly than it currently does. The Earth’s gravity caused part of the Moon to bulge out. The pull of gravity caused the rotation of the Moon to slow down until this bulge was pointing directly at the Earth. At this point, the Moon was tidally locked to the Earth; this is why it shows the same face to us.
And it’s also why a lunar day lasts the same as it takes the Moon to go around the Earth.
One of the most famous pictures taken during the space age is Earthrise, captured by the Apollo 8 astronauts. Here’s an article about it, and here’s an update from the Japanese Kaguya spacecraft.
A NASA astronaut on the lunar surface (credit: NASA)
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Are you feeling heavy? Maybe it’s time to go to the Moon, where you’ll experience much less gravity. Since the Moon is smaller, and has much less mass, it pulls with less gravity. In fact, if you could stand on the surface of the Moon, you would experience only 17% the force of gravity that you would experience on Earth. Gravity on the Moon is much less.
Just to give you an example, let’s say that you weighed 100 kg on Earth. If you stood on the Moon, and then onto your bathroom scale your weight would only be 17 kg. With gravity on the Moon so low, you would be able to jump much higher. If you can jump 30 cm on Earth, you would be able to jump almost 2 meters straight up into the air. And you would be able to fall much further on the Moon. If you jumped off the roof of your house, it would only feel like you jumped off a table. You would be able to throw a ball 6 times further, hit a golf ball 6 times further… you get the idea.
When the Apollo astronauts first walked on the surface of the Moon, they needed to learn how to walk differently in the Moon’s gravity. That’s why the astronauts do a funny hopping run as they move across the surface of the Moon. If they tried to take normal steps, they would fly up into the air to far and fall over – that did happen a few times.
One last, fascinating idea. The pull of gravity on the Moon is so low that you could actually fly with wings attached to your arms (as long as you were inside an enclosed dome filled with air at the Earth’s atmospheric pressure. Wouldn’t it be great to be able to fly around like a bird?
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Have you ever heard that there’s a special time of the year when you’ll be able to see Mars in the sky so big that it looks like a double Moon? You might have gotten this as an email from a friend or family member. Here’s an example of the email.
The Red Planet is about to be spectacular! This month and next, Earth is catching up with Mars in an encounter that will culminate in the closest approach between the two planets in recorded history. The next time Mars may come this close is in 2287. Due to the way Jupiter’s gravity tugs on Mars and perturbs its orbit, astronomers can only be certain that Mars has not come this close to Earth in the Last 5,000 years, but it may be as long as 60,000 years before it happens again.
The encounter will culminate on August 27th when Mars comes to within 34,649,589 miles of Earth and will be (next to the moon) the brightest object in the night sky. It will attain a magnitude of -2.9 and will appear 25.11 arc seconds wide. At a modest 75-power magnification
Mars will look as large as the full moon to the naked eye. By the end of August when the two planets are closest, Mars will rise at nightfall and reach its highest point in the sky at 12:30 a.m. That’s pretty convenient to see something that no human being has seen in recorded history. So, mark your calendar at the beginning of August to see Mars grow progressively brighter and brighter throughout the month. Share this with your children and grandchildren. NO ONE ALIVE TODAY WILL EVER SEE THIS AGAIN
Are we going to get a chance to see a double Moon? I’m sorry, but this is a complete hoax and Internet myth. We’ve written many times about this on Universe Today. Here’s a link to a more complete article.
Each time this email hoax goes around the Internet, it doesn’t mention the year. It only says August 27th, but it doesn’t say what year. In reality, this email first started in 2003. But because the email doesn’t have a year, it keeps coming around year after year. There wasn’t a double moon back in 2003. And there won’t be one this year – whenever you’re reading this.
Mars did make a close approach back in 2003, but it was only slightly closer than it gets any other year that it makes a close approach to the Earth. It came within 34.6 million km. But if you don’t understand how far away that is, it’s hard to see that it can’t be anywhere near as close or big as the Moon. Mars looked like a bright red star in the sky. But nothing like a double moon.
What this email is trying to say is that if you put your eye to the telescope and looked at Mars at 75 power magnification, it would look about the same size as the Moon looks with the unaided eye. In other words, you’d see a double moon if you could somehow look at both at the same time – but you can’t.
I hope this helps clear up the double moon myth.
We’ve tackled this myth many times in the past. Here’s the one we did in 2006, 2007, and 2008.
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The astronomical symbol for the Moon is easy to recognize: it’s a crescent moon. Both the crescent and decrescent moon symbols are used to represent the Moon in astronomy, astrology and alchemy.
When the crescent is on the right, this is the first phase of the Moon, as seen by the northern hemisphere. Think about that for a second, when you’re standing in the southern hemisphere, your view of the Moon is reversed. So from a southern perspective, the crescent will be on the left. But for people in the northern hemisphere, when the crescent is on the right, it’s the first quarter, just after the new moon. And when the crescent is on the left, it’s in the last quarter, just before the new Moon.
Calendars often use a different set of symbols for the Moon to designate the different phases.
Full Moon
First Quarter
Last Quarter
New Moon
This is the same symbol used for the Moon in astrology, and represents silver in alchemy.
Want to know more symbols, here’s the symbol for the Sun, and here’s the symbol for the Earth.
Here’s more information about the Moon symbol from symbols.com.
A species of life on Earth could possibly survive on Enceladus. Credit: JPL
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The discovery of solitary little critters deep beneath Earth’s surface has set the world of microbiology on its head while exciting astrobiologists about the possibility of life on other planets. A community of bacteria was found 2.8 kilometers below ground in a goldmine and it lives completely alone and completely independent of any other life forms. It also subsists without sunlight or oxygen. Planetary scientist Chris McKay, of NASA’s Ames Research Center says that the species Candidatus Desulforudis audaxviator is an amazing discovery, and represents the kind or organism that could survive below the surface of Mars or Saturn’s sixth largest moon Enceladus.
Nicknamed “the bold traveler,” the species was found in fluid-filled cracks of the Mponeng goldmine in South Africa. The discovery of the species contradicts the principle that all life on earth is part of one great, interdependent system.
Scientists extracted all of the DNA present within 5,600 liters of fluid from a fracture deep within the mine. Expecting to find a mix of species within the fluid, the researchers were surprised to find that 99.9% of the DNA belonged to one bacterium, a new species. The remaining DNA was contamination from the mine and the laboratory.
A community of a single species is almost unheard of in the microbial world. But this little bacteria has been happily living on its own and seems to have all of the genetic machinery to enable it to survive independently. Since it is the only species in the ecosystem, it must extract everything it needs from an otherwise dead environment.
Analysis by Dylan Chivian of the Lawrence Berkeley National Laboratory showed that D. audaxviator gets its energy from the radioactive decay of uranium in the surrounding rocks. It has genes to extract carbon from dissolved carbon dioxide and other genes to fix nitrogen, which comes from the surrounding rocks. Both carbon and nitrogen are essential building blocks for life as we know it.
D. audaxviator can also protect itself from environmental hazards by forming endospores – tough shells that protect its DNA and RNA from drying out, toxic chemicals and from starvation. It has a flagellum to help it navigate.
Every other species that we know of on Earth planet relies on other species for some benefit. For example, humans rely on plants to photosynthesize so that we can eat them. Also, other known ecosystems on Earth that don’t use sunlight directly, such as lifeforms found in deep sea vents, do use some form of photosynthesis. But this newly found species actually can’t handle oxygen
The water in which D. audaxviator lives has not seen the light of day in over 3 million years, and this could be an indication of how old the species is.
When we start to look for life on other planets, the discovery of this species will help broaden the horizons of our search.
Earth and Moon, seen from Mars. Image credit: NASA
The diameter of the Moon is 3,474 km. (Diameter of the Moon in miles: 2,159 miles)
Need to put this in context? The diameter of the Earth is 12,742 km, so the Moon’s diameter is about 1/4 that of the Earth (27.3% to be exact). Need another comparison? The diameter of Jupiter’s moon Ganymede – the largest moon in the Solar System – is 5,268 km across. This makes it about 1.5 times larger than the Moon. The Moon is the only natural satellite of the Earth, and the fifth largest moon in the Solar System.
Like most objects in the Solar System, the Moon spins on its axis, completing a day in 27.3 Earth days. Because it’s rotating, the Moon slightly flattens out. I say slightly, because we’re going to need decimals to really tell the difference. The equatorial diameter of the Moon is 3,476.28 km. And the polar diameter of the Moon is 3,471.94 km. In other words, the Moon’s diameter from side to side is 4.34 km more than its distance from pole to pole.
These two previously released infrared images of Saturn show the entire south polar region with the hurricane-like vortex in the center. The top image shows the polar region in false color, with red, green, and blue depicting the appearance of the pole in three different near-infrared colors (NASA/JPL/University of Arizona)
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The Cassini mission has released some of the most detailed images of Saturn’s poles yet, revealing vast cyclones churning up the gas giant’s atmosphere in the north and south. These observations show very similar storms to the south pole observations imaged by the NASA spacecraft in 2006, only in far better detail. It is believed the north and south cyclones are generated by violent thunderstorms deep inside Saturn’s atmosphere; water condensing inside these storms output heat, fuelling the vortex extending 2,500 miles (4,000 kilometres) in diameter. The smallest features resolved are 120 kilometre (75 mile)-wide cumulus clouds rotating at velocities in excess of 325 mph (530 kph), more than twice the wind speed possible on Earth…
…and the mystery north pole hexagon is still there.
Images of the large cyclones at the north (left) and south (right) poles of Saturn taken in June 2008 by the visual and infrared mapping spectrometer onboard the Cassini spacecraft (NASA/JPL/University of Arizona)Cassini has wowed scientists with these brand new views of Saturn’s north pole. With detail at 10-times higher resolution than previously attained, Cassini has shown that both poles have vast swirling cyclones that highlight regions of planet-wide storm activity.
“These are truly massive cyclones, hundreds of times stronger than the most giant hurricanes on Earth,” said Kevin Baines, Cassini scientist on the visual and infrared mapping spectrometer at NASA’s Jet Propulsion Laboratory. “Dozens of puffy, convectively formed cumulus clouds swirl around both poles, betraying the presence of giant thunderstorms lurking beneath. Thunderstorms are the likely engine for these giant weather systems.”
The Saturn hexagon as seen by Voyager 1 in 1980 (NASA)Interestingly, the northern storm observation still shows the mysterious hexagonal shape (as originally verified in 2006, after a sighting by Voyager 1 in 1980), only in far greater detail. Scientists are still uncertain why the northern cyclone should take such a stable form; the clouds within the hexagonal shape spin at high speeds without interfering with its six-sided shape.
Previous observations appeared to show an outer ring of high clouds surrounding a region thought to be clear air with a few puffy clouds circulating around the pole. These brand new images reveal a far more complex picture. The circulating clouds are actually smaller convective storms forming other, more distinct rings.
Oblique view of Saturns south polar vortex (NASA)
Tony DelGenio of NASA’s Goddard Institute for Space Studies in New York and Cassini imaging team member explains the scene: “What looked like puffy clouds in lower resolution images [from 2006] are turning out to be deep convective structures seen through the atmospheric haze. One of them has punched through to a higher altitude and created its own little vortex.”
These clouds push high above the main weather system, casting long shadows, indicating they are 25-45 miles (40-70 km) above the vortex rings (pictured above). These also appears to be an inner ring of clear air (the “eye” of the storm) over the poles, that appeared bigger in previous observations.
For me, the most perplexing feature to come out of these new Cassini observations is the enduring hexagonal shape in the circulating clouds. In 2006 when the lower-resolution north pole observations were released, many made the assumption that it was an unstable transient feature, appearing for a short period, only to disappear soon afterwards. But over a year later, the hexagon remains, as six-sided as ever. I’d guess this shape could be some atmospheric standing wave, what do you think?
See the Cassini video of the north pole hexagon being buffetted by the high-speed winds surrounding it »
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If the Moon’s up, go take a look and see what color it is. If you’re looking during the daylight, the Moon will look faint and white surrounded by the blue of the sky. If it’s night, the Moon will look bright yellow. Why does the color the Moon seem to change from white to yellow when you go from day to night. And why does the Moon look gray in many photographs, especially the ones from space? What color is the Moon?
The photographs of the Moon, taken from space are the best true-color views of the Moon. That gray color you see comes from the surface of the Moon which is mostly oxygen, silicon, magnesium, iron, calcium and aluminum. The lighter color rocks are usually plagioclase feldspar, while the darker rocks are pyroxene. Most of the rocks that you can see are volcanic, and were extruded from the inside of the Moon during volcanic eruptions. Some rare rocks called olivine are actually green.
The dark regions you see on the Moon are called lunar maria, and they were formed by ancient volcanic eruptions. They’re less reflective than the lunar highlands, and so they appear darker to the eye. The maria cover about 16% of the lunar surface, mostly on the side we can see from Earth. Astronomers think the lunar maria were formed about 3-3.5 billion years ago, when the Moon was much more volcanically active.
When you see the Moon from here on Earth, the atmosphere partially blocks your view. The particles in the atmosphere scatter certain wavelengths of light, and permit other wavelengths to get through directly. When the Moon is low in the sky, you’re seeing its light go through the most atmosphere. Light on the blue end of the spectrum is scattered away, while the red light isn’t scattered. This is why the Moon looks more red. As it goes higher in the sky, the Moon is obscured by less and less atmosphere, so it turns more yellow – the same thing happens to the Sun as it rises in the sky.
During the day, the Moon has to compete with sunlight, which is also being scattered by the atmosphere, so it looks white.
Here’s an article that explains how to get the right color of the Moon in Photoshop, and here’s an article from Windows on the Universe about the Moon’s colors in fall.
Astronauts need spacesuits to survive the temperature of the Moon. Image credit: NASA
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Are you planning a trip to the Moon and you’re wondering what kinds of temperature you might experience. Well, you’re going to want to pack something to keep you warm, since the temperature of the Moon can dip down to -153°C during the night. Oh, but you’re going to want to keep some cool weather clothes too, since the temperature of the Moon in the day can rise to 107°C.
Why does the moon’s temperature vary so widely? It happens because the Moon doesn’t have an atmosphere like the Earth. Here on Earth, the atmosphere acts like a blanket, trapping heat. Sunlight passes through the atmosphere, and warms up the ground. The energy is emitted by the ground as infrared radiation, but it can’t escape through the atmosphere again easily so the planet warms up. Nights are colder than days, but it’s nothing like the Moon.
There’s another problem. The moon takes 27 days to rotate once on its axis. So any place on the surface of the Moon experiences about 13 days of sunlight, followed by 13 days of darkness. So if you were standing on the surface of the Moon in sunlight, the temperature would be hot enough to boil water. And then the Sun would go down, and the temperature would drop 250 degrees in just a matter of moments.
To deal with this dramatic range in temperature, spacesuits are heavily insulated with layers of fabric and then covered with reflective outer layers. This minimizes the temperature differences between when the astronaut is in the sunlight and when in shade. Space suits also have internal heaters and cooling systems, and liquid heat exchange pumps that remove excess heat.
There are craters around the north and south poles of the Moon which are bathed in complete shadow, and never see sunlight. This places would always be as cool as -153°C. Similarly, there are nearby mountain peaks which are bathed in continuous sunlight, and would always be hot.
We have written many articles for Universe Today about some of the special regions of the Moon. Here’s an article about building a moon base, and here’s an article about a perfect crater for a human settlement.
The astronomy world buzzed in the Fall of 2007 when Comet Holmes – a normally humdrum, run-of-the-mill comet — unexpectedly flared and erupted. Its coma of gas and dust expanded away from the comet, extending to a volume larger than the Sun. Professional and amateur astronomers around the world turned their telescopes toward the spectacular event. Everyone wanted to know why the comet had suddenly exploded. The Hubble Space Telescope observed the comet, but provided few clues. And now, observations taken of the comet after the explosion by NASA’s Spitzer Space Telescope deepen the mystery, showing oddly behaving streamers in the shell of dust surrounding the nucleus of the comet. The data also offer a rare look at the material liberated from within the nucleus. “The data we got from Spitzer do not look like anything we typically see when looking at comets,” said Bill Reach of NASA’s Spitzer Science Center at Caltech.
Every six years, comet 17P/Holmes speeds away from Jupiter and heads inward toward the sun, traveling the same route typically without incident. However, twice in the last 116 years, in November 1892 and October 2007, comet Holmes exploded as it approached the asteroid belt, and brightened a millionfold overnight.
In an attempt to understand these odd occurrences, astronomers pointed NASA’s Spitzer Space Telescope at the comet in November 2007 and March 2008. By using Spitzer’s infrared spectrograph instrument, Reach and his colleagues were able to gain valuable insights into the composition of Holmes’ solid interior. Like a prism spreading visible-light into a rainbow, the spectrograph breaks up infrared light from the comet into its component parts, revealing the fingerprints of various chemicals. The Spitzer Space Telescope. Credit: NASA
In November of 2007, Reach noticed a lot of fine silicate dust, or crystallized grains smaller than sand, like crushed gems. He noted that this particular observation revealed materials similar to those seen around other comets where grains have been treated violently, including NASA’s Deep Impact mission, which smashed a projectile into comet Tempel 1; NASA’s Stardust mission, which swept particles from comet Wild 2 into a collector at 13,000 miles per hour (21,000 kilometers per hour), and the outburst of comet Hale-Bopp in 1995.
“Comet dust is very sensitive, meaning that the grains are very easily destroyed, said Reach. “We think the fine silicates are produced in these violent events by the destruction of larger particles originating inside the comet nucleus.”
When Spitzer observed the same portion of the comet again in March 2008, the fine-grained silicate dust was gone and only larger particles were present. “The March observation tells us that there is a very small window for studying composition of comet dust after a violent event like comet Holmes’ outburst,” said Reach.
Comet Holmes not only has unusual dusty components, it also does not look like a typical comet. According to Jeremie Vaubaillon, a colleague of Reach’s at Caltech, pictures snapped from the ground shortly after the outburst revealed streamers in the shell of dust surrounding the comet. Scientists suspect they were produced after the explosion by fragments escaping the comet’s nucleus.
In November 2007, the streamers pointed away from the sun, which seemed natural because scientists believed that radiation from the sun was pushing these fragments straight back. However, when Spitzer imaged the same streamers in March 2008, they were surprised to find them still pointing in the same direction as five months before, even though the comet had moved and sunlight was arriving from a different location. “We have never seen anything like this in a comet before. The extended shape still needs to be fully understood,” said Vaubaillon.
He notes that the shell surrounding the comet also acts peculiarly. The shape of the shell did not change as expected from November 2007 to March 2008. Vaubaillon said this is because the dust grains seen in March 2008 are relatively large, approximately one millimeter in size, and thus harder to move.
“If the shell was comprised of smaller dust grains, it would have changed as the orientation of the sun changes with time,” said Vaubaillon. “This Spitzer image is very unique. No other telescope has seen comet Holmes in this much detail, five months after the explosion.”
“Like people, all comets are a little different. We’ve been studying comets for hundreds of years — 116 years in the case of comet Holmes — but still do not really understand them,” said Reach. “However, with the Spitzer observations and data from other telescopes, we are getting closer.”
Full Moon 
First Quarter
Last Quarter
New Moon
