Earth Science Picture of the Day

I had the Earth Science Picture of the Day for today, Thursday, July 28.  Cool!

http://epod.usra.edu/blog/2011/07/upheaval-dome.html

I'll get back to blogging soon, have been taking a short hiatus.

Moon or Afghanistan

Taking a one day break from my Movements of the Moon series.

On a semi-related note, here's a true and depressing cartoon from The Pain Comics.  Fucking al Quaida.

Movements of the Moon - Part V

Links to Part I, Part II, Part III, and Part IV of this series.

Far, far, far away from our little solar system are the visible stars of our local neighborhood in the Milky Way galaxy.  As you go outside at night, and look up at the starry sky (to be fully human, you should do this periodically), you'll see random scatterings of stars and your mind will automatically want to arrange them in patterns.  Humans have done this since time immemorial and created what we today know as constellations.  Modern astronomy recognizes 88 constellations (some we can't see here in the Northern Hemisphere), most named from ancient Greek mythology.

All of the planets basically orbit the Sun in the same plane which is called the ecliptic.

When we look up at the night sky, any planets, if they're visible, will always lie along the ecliptic.  Here's what you would have seen in Ulster County, for example, in early May, 2002 just after sunset.

The red line is the ecliptic and the five naked-eye planets were visible in the sky that evening - Mercury, Venus, Mars, Jupiter, and Saturn.  You always look for the planets in the region of the sky where the ecliptic runs; you'd never look for a planet in the constellation of Orion, for example, because the ecliptic doesn't run through Orion.

The part of the sky where the ecliptic occurs was long ago (we're talking ancient cultures of Meospotamia thousands of years ago) divided up into roughly equal regions signified by 12 distinct constellations - Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces.  The zodiac (modern astronomers recognize these constellations today even though they totally reject their astrological roots.

We call this the zodiac, derived from the Greek ζῳδιακὸς κύκλος (zōdiakos kuklos) or "circle of animals".  During different parts of the year, we see different zodiacal constellations in the night sky because we're in different parts of the Earth's orbit.

OK, what does this have to do with the Moon?  Well the Moon orbits the Earth a few degrees off the ecliptic (we'll talk more about this in another post) so the Moon also appears each night in one of the zodiacal constellations.

As an example, the following figures (click to enlarge) show the position (and phase) of the Moon on July 10, 12, 14, 16, and 18 of 2011 in the southern sky as seen from Ulster County, NY at midnight.  The red line is the ecliptic, the plane in which all the planets orbit the Sun (almost).  Note that the Moon moves from slightly below the ecliptic to slightly above the ecliptic for reasons we'll talk about another day.

 

 

Over the course of about a week the Moon moves through the zodiacal constellations of Libra, Scorpius, Sagittarius, Capricornus, and Aquarius.  Over the course of a sidereal month (one orbit of the Moon around the Earth with respect to the distant stars), the Moon will move through all 12 of the zodiacal constellations.  We can't see it in some of them because when the Moon is out during the day (near its New Moon phase), we can't see the stars.  On August 1, 2011, for example, the Moon will be in Leo but rising and setting with the Sun so completely invisible.

Some ancient cultures kept calendar time by the Moon's 27.32 day sidereal movement through the zodiac instead of the by the 29.52 day synodic cycle of phases.  Ancient Chinese astronomers, for example, divided the zodiac into four regions - azure dragon, black tortoise, white tiger, and vermilion bird.  Each of these was then subdivided into 7 "mansions" giving a total of 28 "mansions".  The Moon basically passes through a lunar mansion each day.

Next time, we'll talk about the rising and setting times of the Moon.

Movements of the Moon - Part IV

If you haven't already, read Part I, Part II, and Part III of this series first.

To date, we've now discussed phases of the Moon, the synodic month (a lunation or cycle of phases), and the sidereal month (an orbit of the Moon around the Earth).  Today, I'd like to discuss a couple of miscellaneous concepts.

The image at right shows the phases of the Moon from Waxing Crescent, 1st Quarter, Waxing Gibbous, to Full Moon and then back to Waning Gibbous, 3rd or Last Quarter, Waning Crescent, and New Moon (the blank space in the lower-right corner).

Note that the features you see on the face of the Moon never change.  In other words, the same side of the Moon is always facing the Earth no matter what the phase.  This is called synchronous rotation and is due, of course, to the strong gravitational attraction between the Earth and Moon.

The side always facing away from the Earth is called the far side and wasn't even seen by humans until imaged by the Russian Luna 3 probe in 1959.  As I mentioned in a previous post in this series, it's sometimes called the dark side but the word "dark" is used only in the sense of it being unknown (at the time), not an absence of light.

The so-called dark side of the Moon is fully lit by the Sun for two weeks each month.  A New Moon phase, when the Moon is between the Earth and the Sun, has the far side of the Moon in full sunlight while the near side that faces the Earth is in darkness (as illustrated by our old friend, the diagram below).

The interesting thing about the far side of the Moon, is that it's very different in appearance from the near side.  The near side has more mare (pronounced "mar-ey" - Latin for "seas") - large volcanic lava plains - and the far side is heavily cratered.  I'll discuss why another time.

Another interesting fact about this tidal locking of the Earth and the Moon is that the Moon "rocks" back and forth a little bit in its orbit allowing us to actually see a bit more than half (59%) of its surface from the Earth.  This is called lunar libration.  The image below illustrates this, both show photos of the waxing Moon at two different times.  The red dot denotes Mare Crisium ("Sea of Crisis") near the Moon's eastern edge.  See the slight difference in position from lunar libration?

Speaking of the size of the Moon, perhaps I should say a few words about the Moon illusion.  When a Full Moon is rising on the eastern horizon it often looks huge - much larger than it appears when higher in the sky.

It's an optical illusion.  You can photograph the rising Moon and establish that it's size really doesn't change - it just appears large near the horizon (the color change is due to blue wavelengths being scattered out near the horizon - leaving red wavelengths to get through - since you're looking though a thicker layer of the atmosphere).

The Moon is actually surprisingly small in the sky.  It covers 1/2° - for reference, the distance from the horizon to the zenith (the point directly over your head) is 90°.  That's means you hold your arm outstretched, extend your pinkie finger, and the fingernail on your pinkie will completely cover the Moon (even when it's on the horizon).  Don't believe me?  Go out at night and try it.

Why do we perceive the Moon larger on the horizon?  No one's quite sure (or, maybe I should say, lots of people think they're sure but they offer competing hypotheses).  Here's a summary of ideas from Dr. Donald Simanek, a retired physics professor.

Enough for today.  The topic for tomorrow, which I briefly mentioned yesterday, goes back to the sidereal month and the Moon moving through the constellations.