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moon

a waning “C” crescent and a waxing “D” crescent

I was out last night with a friend who commented that there was a clear “crescent Moon.” People commonly use that term when a sliver of Moon is showing, but there are two versions of the crescent sliver.

The Moon is always waxing (growing in the lit area we see) and waning, and moving closer and farther away from us. It is surprising how many people have never really noticed that the Moon looks like a looks like a “C” crescent, and later looks like a “D” in its waxing phase.

moon phases

The phases of the Moon as viewed looking southward from the Northern Hemisphere. Each phase would be rotated 180° if seen looking northward from the Southern Hemisphere. The upper part of the diagram is not to scale, as the Moon is much farther from Earth than shown here.

In Hinduism, every part of the cosmos is seen as an action of a god and time is the endless repetition of the same long cycle. In Hindu mythology, Soma represents the god of the Moon.

Soma rides a sky chariot drawn by white horses. Soma was also the name of the elixir of immortality that only the gods can drink. The elixir is stored on the Moon. When the gods drink soma, they draw away from the Moon and it becomes smaller. (I wrote about soma earlier in another context.)

Most people know that the Moon changes its distance from Earth continually because the orbit of the moon is not a perfect circle. It is more like an ellipse, so it will have a point of perigee (closest point to Earth) and apogee (farthest point) each month. Today, May 6, it is at apogee and it is 251,318 miles or 404,457 km away from us.

Back on April 20 perigee, it was  229,108 miles or 368,714 km away. In cosmic terms, a difference of 22,210 miles or 35,743 km is not that much and only astronomers take note of the diference. But occasionally the media will decide to write a story about the “biggest Full Moon of the year” or something similar.

There is a nice animation at time.unitarium.com/moon/ that shows the movement of the Moon in your area and illustrates nicely why we see a Full Moon and how it appears when waxing and waning.  You can set it to any date, so I know that on my next October birthday the Moon will be waxing gibbous and approaching full. Unfortunately, it doesn’t allow you to go back before 2000 or I would take a look at what the Moon was up to when I was born.

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When I really started paying attention to the Sun, stars and Moon many years ago, one of the things that confused me was why the Quarter Moons looked like Half Moons.

The Moon is at or near its last quarter phase tonight, February 6, and into tomorrow morning. (The precise time is tomorrow at 15:54.)  Take a look tonight and you will see half of the Moon. Half the moon always faces us, and half the moon is always lit by the sun, though we can’t see that. To astronomers, there are no ‘half moons.’

So why does this phase get the name Quarter Moon is we can see half of it lit?  First quarter moon means the moon is one-quarter of the way through the current orbital cycle. Tonight’s third or last quarter moon means the moon is three-quarters of the way through the cycle, as measured from one new moon to the next.

moon phases

The phases of the Moon as viewed looking southward from the Northern Hemisphere. Each phase would be rotated 180° if seen looking northward from the Southern Hemisphere. The upper part of the diagram is not to scale, as the Moon is much farther from Earth than shown here. Image: wikipedia.org

It is all about perspective. At first quarter moon, the near side of the moon (the part we see) is half-illuminated by sunlight and half in its own shadow, so we are seeing half the moon’s day side.

It may also seem curious that in the Southern Hemisphere tonight the right side is 50% lit and in my Northern Hemisphere it is the left side that is bright. Yes, when we enter the first quarter that will be reversed.

A third quarter moon always rises in the middle of the night. It will appear at its highest in the sky around dawn, and will set around midday.


To move away from the sky though, I do like the name “Half Moon.” Half Moon Bay is a town on the California coast that I visited once and the name seems kind of romantic. There is a song called “Half Moon Bay” from 1969 by a band that I followed, Mott the Hoople. The lyrics have nothing to do with the town and the Dylan-esque vocals don’t make the lyrics any happier or romantic. It was a song I liked for its Procol Harum-like organ back then – and the album’s Escher cover is still a favorite. (Listen  on YouTube)

Another more recent song with that same title is by Train.  This one is actually about the California town and more “romantic.”

This ain’t a threat but I think I better warn ya’
Gonna fall in love if you go to California
I did and this is how I know
By the beach north of San José
Met the right girl and it sounds cliché
But we decided not to take it slow

But remember, there may be a Half Moon Bay, but there are no half moons.

Tomorrow, will be the New Moon when the portion of the moon we see from Earth is 0% illuminated by sunshine and so completely engulfed in the moon’s own shadow.

The moon has a day side and a night side, just as Earth does. Due to the angle between the sun, Earth and moon, we see different portions of its day side and night side as the moon phases progress. When it is waxing, we see more of its day side at night until it turns full.

The part of the moon that isn’t in sunlight is often called the “dark side of the moon.” There is a famous record album with that name. But because of the moon’s motion around Earth, the “night side of the moon” that we see from Earth constantly changes. That means there is a permanent far side of the moon, but there is no permanent dark side of the moon.

If you were to spend time in one place on the Moon, you would experience night for about two weeks, followed by about two weeks of daylight.

After billions of years of Earth’s strong gravitational pull, the Moon has actually slowed down so that it takes as long to rotate as it does to orbit once around Earth. It is “tidally locked” with Earth.

But “tidally locked” and the “far side of the moon” do not sound right for a song lyric, and Pink Floyd do say in that album’s final track, “Eclipse

And all that is now
And all that is gone
And all that’s to come
And everything under the sun is in tune
But the sun is eclipsed by the moon

There is no dark side in the moon really
Matter of fact it’s all dark

Tonight the Moon will look like a “half-moon” but it is officially at the First Quarter. It will be 45 percent visible to us, which certainly sounds like about a half-moon. About half of it will be illuminated by direct sunlight, but it will be only “7 days old” in its waxing growth from the New Moon when its unilluminated side was facing the Earth. So, it is a quarter of the way into its cycle of phases from new to full.

moon 4 phases

Quick Review

The Waxing (“increasing”)  crescent is when the Moon appears to be partly but less than one-half illuminated by direct sunlight.

First Quarter is when it looks like half a circle because it has completed one-quarter of an orbit around the Earth from either the full or new position.

Waxing Gibbous is when it appears to be more than one-half but not fully illuminated by direct sunlight.

The most popular phase, the Full Moon, is when the entire illuminated side is facing the Earth.

And then as we see less of it from Earth, the Waning Gibbous appears to be more than one-half but not fully illuminated by direct sunlight.

The Last Quarter is that other half of the Moon being illuminated by direct sunlight, and the Waning Crescent is the other side of the Waxing Crescent.

A nice site to see all the phases on a calendar is MoonGiant.com

Some of you might be surprised to see the Moon out this morning or for the next few days. It’s not a rare thing but because we so strongly associate the Moon with nighttime, it always seems to surprise some people.

Sometimes this view is called the Children’s Moon because it is one time young children who go to bed too early to see the Moon at night can still see it in the sky. If you want to explain it to a child (okay, so maybe it’s an adult), here is some information.

Pretty much anywhere on the planet now looking generally westward after sunrise with a clear blue daytime sky, you can spot the Moon. We just had the Full Moon 3 days ago, so it’s now in a waning gibbous phase and it “rises” after nightfall and “sets” to the west after sunrise, so you see it in the morning sky. It will be climbing higher and higher into the sky each day for the next week.

When the Moon moves into its last quarter phase in four days, it will rise at midnight and set southward around dawn. By the time we reach the New Moon on the 16th, it will moving across the sky with the sun and so not visible in our sky.

More at www.space.com

 

new moon smallToday, May 18, is the day of the New Moon. Some people think of this as the Dark Moon or “No Moon.”

In one of the many celestial plays of motion and opposites, this is when the Moon transitions from the morning to the evening sky. Today, and at this point every month, the Sun and Moon lie on the same side of Earth in space.

The Moon rises with the Sun at sunrise and then crosses the sky with the Sun during the daytime and sets with the Sun at sunset. The New Moon follows the Sun.

For perspective, at the Full Moon, the sun and moon lie on opposite sides of Earth and the Moon will then rise around sunset, cross the sky during the night and set around sunrise.

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