Category: Astronomy

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The mass of the Moon is 7.347 x 10²² kg.

That sounds like a large number, and I suppose it is compared to the mass of a single person, a car or even a building. But you’ve got to keep it in context. The mass of the Moon is only 1.2% the mass of the Earth. In other words, you would need 81 objects with the mass of the Moon to match the mass of the Earth.

The diameter of the Moon is only about 1/4 the diameter of the Earth, so it might seem like the mass of the Moon is strangely low. And you would be right. The key is the Moon’s low density. It has a density of only 3.3 g/cm³. This is almost half the density of Earth.

Astronomers think that a Mars-sized object crashed into the Earth about 100 million years after the Earth formed. The huge cloud of ejected debris coalesced into the Moon, which still orbits us today. The Moon has a lower density because the impact gouged out the outer crust and mantle, and didn’t eject so much of the Earth’s iron core.

Want more information about the mass of the Earth? Or what about the mass of Mars?

The Physics Factbook has more information about the mass of the Moon. And here’s an article that explains how you would go about weighing the Moon.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

Reference:
NASA Moon Facts

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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.

Here’s an animation from NOAA showing how the Moon’s position affects the tides. And have you ever wondered why you can see the Moon during the day?

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

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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?

Do you wonder about the gravity of Mars, or the gravity of Jupiter?

There are some cool calculators out there that let you take your weight and see what you would experience on other planets. Check this one out.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

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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.

Maybe you’re looking for the Double Moon music company?

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

Reference:
NASA: Beware the Mars Hoax

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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.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

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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 from Universe Today about harvest moons, and here’s an article about how astronomers calibrate photographs from space.

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.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

Reference:
http://www.nasa.gov/mission_pages/LRO/multimedia/lroimages/lroc-20100910_color_moon.html

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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.

Here’s an answer to the question from Windows on the Universe, and here’s some information from Teacher’s Domain.

You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?

Reference:
NASA Moon Facts