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Wednesday, 28 November 2018

5 Interesting Facts About The Moon

5 Interesting Facts About The Moon

As the full moon approaches, its growing brightness tends to capture our attention.

The full moon occurs when the moon is on the opposite side of Earth from the sun, so that its face is fully illuminated by the sun's light. [Photos: Our Changing Moon]

But any day of the month, the moon has some secrets up her sleeve. Here are 10 surprising and strange facts about the moon that may surprise you:



1) There's actually four kinds of lunar months

Our months correspond approximately to the length of time it takes our natural satellite to go through a full cycle of phases.  From excavated tally sticks, researchers have deduced that people from as early as the Paleolithic period counted days in relation to the moon's phases. But there are actually four different kinds of lunar months.  The durations listed here are averages.

*     Anomalistic – the length of time it takes the moon to circle the Earth, measured from one perigee (the closest point in its orbit to Earth) to the next: 27 days, 13 hours, 18 minutes, 37.4 seconds.

*     Nodical – the length of time it takes the moon to pass through one of its nodes (where it crosses the plane of the Earth's orbit) and return to it: 27 days, 5 hours, 5 minutes, 35.9 seconds.

*    Sidereal – the length of time it takes the moon to circle the Earth, using the stars as a reference point: 27 days, 7 hours, 43 minutes, 11.5 seconds.

*    Synodical – the length of time it takes the moon to circle the Earth, using the sun as the reference point (that is, the time lapse between two successive conjunctions with the sun – going from new moon to new moon): 29 days, 12 hours, 44 minutes, 2.7 seconds.  It is the synodic month that is the basis of many calendars today and is used to divide the year.

2) It would take hundreds of thousands of moons to equal the brightness of the sun

The full moon shines with a magnitude of -12.7, but the sun is 14 magnitudes brighter, at -26.7.  The ratio of brightness of the sun versus the moon amounts to a difference of 398,110 to 1.  So that's how many full moons you would need to equal the brightness of the sun.  But this all a moot point, because there is no way that you could fit that many full moons in the sky.

The sky is 360 degrees around (including the half we can't see, below the horizon), so there are over 41,200 square degrees in the sky. The moon measures only a half degree across, which gives it an area of only 0.2 square degrees. So you could fill up the entire sky, including the half that lies below our feet,  with 206,264 full moons — and still come up short by 191,836 in the effort to match the brightness of the sun.


3) We see slightly more than half of the moon from Earth

Most reference books will note that because the moon rotates only once during each revolution about the Earth, we never see more than half of its total surface. The truth, however, is that we actually get to see more of it over the course of its elliptical orbit: 59 percent (almost three-fifths). ['Supermoon' Full Moons Explained]

The moon's rate of rotation is uniform but its rate of revolution is not, so we're able to see just around the edge of each limb from time to time.  Put another way, the two motions do not keep perfectly in step, even though they come out together at the end of the month. We call this effect libration of longitude.

So the moon "rocks" in the east and west direction, allowing us to see farther around in longitude at each edge than we otherwise could. The remaining 41 percent can never be seen from our vantage point; and if anyone were on that region of the moon, they would never see the Earth.

4) The first- or last-quarter moon is not one half as bright as a full moon

If the moon's surface were like a perfectly smooth billiard ball, its surface brightness would be the same all over. In such a case, it would indeed appear half as bright.

But the moon has a very rough topography. Especially near and along the day/night line (known as the terminator), the lunar landscape appears riddled with innumerable shadows cast by mountains, boulders and even tiny grains of lunar dust.  Also, the moon's face is splotched with dark regions.  The end result is that at first quarter, the moon appears only one eleventh as bright as when it's full.

The moon is actually a little brighter at first quarter than at last quarter, since at that phase some parts of the moon reflect sunlight better than others.

5) The Earth, seen from the moon, also goes through phases

However, they are opposite to the lunar phases that we see from the Earth. It's a full Earth when it's new moon for us; last-quarter Earth when we're seeing a first-quarter moon; a crescent Earth when we're seeing a gibbous moon, and when the Earth is at new phase we're seeing a full moon.

From any spot on the moon (except on the far side, where you cannot see the Earth), the Earth would always be in the same place in the sky.

From the moon, our Earth appears nearly four times larger than a full moon appears to us, and – depending on the state of our atmosphere – shines anywhere from 45 to 100 times brighter than a full moon.  So when a full (or nearly full) Earth appears in the lunar sky, it illuminates the surrounding lunar landscape with a bluish-gray glow. 

From here on the Earth, we can see that glow when the moon appears to us as a crescent; sunlight illuminates but a sliver of the moon, while the rest of its outline is dimly visible by virtue of earthlight.  Leonardo da Vinci was the first to figure out what that eerie glow appearing on the moon really was.

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