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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.

It gets much more attention when we have a “supermoon,” when the Moon’s orbit is close to Earth, but tonight’s full Moon is the smallest full moon of the year.

Unofficial names for it are micro-moon or mini-moon. Tonight the Moon will be about 50,000 kilometers (30,000 miles) farther away from Earth than will the year’s closest full Moon. The Moon is both full and at lunar apogee (the moon’s farthest point in its monthly orbit) on this day.

For the precisely-minded reader, the crest of the moon’s full phase comes at 18:05 Universal Time, which in Paradelle is during the day at 1:05 p.m. EST, so you might feel its pull under your feet from the other side of the planet.

I’ll remind you come September 28, 2015 that the Full Moon (usually called the Harvest Moon) will be the closest full moon of this year and it will stage a total eclipse of the Moon. These astronomical oddities always get people to look up at the sky with a bit more attention. You’ll probably hear it called a Blood Moon eclipse.

Astronomy.fm Amateur Astronomy Picture of the Day, October 28 2011, Perigeo/Apogeo

Moon at perigee was photographed on March 20,2011 when the Moon (Super Moon) was the largest in nearly twenty years. This is the point in the orbit of the moon or a satellite at which it is nearest to the earth.

The Full Moon was photographed in apogee on October 12 (Columbus Day) at dawn, at the farthest point of its elliptical orbit this year.

Both pictures of the Moon have been captured as it passes through the meridian.

Since the Moon’s sidereal period differs from its synodic period, the perigee of the Moon (the point where it is closest to the Earth) does not stay in sync with the phases of the Moon. So, for example, the “Hunter’s Moon” of October does not correspond to any special timing of the Moon’s distance from the Earth. This is why the Hunter’s Moon is not, in general, brighter than any other regular full moon, though it is often perceived that way.

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