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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.

supermoon

There has been a lot of popular media coverage of the “supermoons” the past few years.  That is not a scientific term, but one coined about thirty years ago. It is more on the side of astrologers than astronomers.

People know that the Moon affects tides. Many people suspect that it affects people and animals, although much of that falls into my “moon lore” category.

But it is great that media coverage pf the term has people looking up at the night sky.  Tonight’s Full Moon will appear about 14% larger than usual at a point in its journey across our sky.

A “supermoon” can be a New Moon or a Full Moon when it occurs at roughly the same time the moon is nearest Earth in its monthly orbit. That is properly called perigee, the term for the moon being at its closest point to Earth.

The term supermoon denotes a new or full moon that occurs at roughly the same time the moon is nearest Earth in its monthly orbit. Astronomers use the term perigee to describe the moon’s closest point to Earth, from the Greek words peri meaning “near” and gee meaning “Earth.”

Our first supermoon of 2014 occurs on the very first day of the year. There will be a second supermoon on January 30.  We won’t have a single calendar month with two supermoons again until January 2018.

“Supermoon” is not a term from astronomy, but from astrology. The belief is that a supermoon has some kind of effect on people on Earth.  It certainly has the effect of high tides.

“Supermoon” is a term that was coined by Richard Nolle over 30 years ago, but it has become more popular in our Internet age.

2014 will have a total of five supermoons: the two new moons of January, and the full moons of July, August and September.

img-sprmoon

“Super Moon” photographed by Tim McCord of Entiat, Washington from March 19, 2011 full moon using a camera-equipped telescope.

I am always a bit surprised by the number of posts and mentions on the news each year when the Moon reaches its perigee.  Some people will call last night’s Full Moon the biggest full moon of the year because it coincides with the moon’s perigee — its closest approach to Earth — and so it will appear bigger. That’s why you may have heard a news story about tonight’s SuperMoon. (If you missed it last night or had a cloudy sky, it will be very similar tonight.)

It will still be 221,802 miles (356,955 kilometers) from our planet, but that is closer, so bigger and brighter (if you have a clear night in your area) gets a lot of attention.

The Greek prefix “peri” means close or near. The suffix “gee”, derived from Gaea, means Earth.

It’s nice that the Moon gets some attention and that some people who never look to the night sky will do so tonight.

Perigee is part of the broader family of “apses”, astronomical terms which denote distances of orbiting bodies. Since all orbits are elliptical, each orbit contains both a nearest point and a farthest point. The opposite is the apogee, the farthest or highest point. That event doesn’t get any press at all.

The name SuperMoon was coined by astrologer Richard Nolle in 1979, who defined it as: “a new or full moon which occurs with the Moon at or near (within 90% of) its closest approach to Earth in a given orbit (perigee). In short, Earth, Moon and Sun are all in a line, with Moon in its nearest approach to Earth.”  Members of  scientific community prefer the term perigee-syzygy.

Some people claim that in the days before and after a supermoon, the Earth is more subject to natural disasters such as earthquakes and volcanic activity due to the Moon’s increased gravitational force. But most scientists and records indicate that any claim of a supermoon effect is unjustified.

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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