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It's Just a Phase - The Moon
updated: Mar 22, 2014, 11:00 AM

By Chuck McPartlin

As the Moon orbits the Earth, and the Earth and Moon travel together around the Sun, we get to see orbital mechanics in action. Every month (think moonth), we are treated to a beautiful sequence of phases as the illumination of the Moon's surface changes. Because we're dealing with multiple celestial bodies with multiple motions like rotation and revolution, it may not be obvious what's really going on. This leads to misconceptions like a permanently "Dark Side of the Moon", the Moon is "only visible at night", or that the lunar phases are caused by the shadow of the Earth. Let's take a look at why we see the Moon the way we do in our Santa Barbara sky.

Let's watch the phases of the Moon as seen from Earth, starting with what has traditionally been called a young or New Moon - the tiny sliver you see in the west near the setting Sun. As the sky darkens, you may see that the rest of the Moon's face is faintly illuminated, often called "the New Moon in the Old Moon's arms". This is Earthshine, light reflected from the bright clouds, snows, and oceans of the Earth.

Over the next five or six days, the brightly illuminated fraction of the visible face of the Moon grows, referred to as the Waxing (growing) Crescent phase. The boundary between light and dark on the Moon is called the terminator, and in these early phases is the line of morning sunrise dawning on the Moon.

About a week after New Moon, the lunar face appears half illuminated, This is First Quarter, when the Moon rises around noontime, and crosses our southern meridian as the Sun is setting. It has moved one quarter of the way around the sky from the Sun. For the next week the illuminated portion exceeds the 50% mark, known as the Waxing Gibbous phase. "Gibbous" comes from Old English for "humpbacked".

The phase of full illumination of the visible surface of the Moon is, as you may suspect, called Full Moon. The Moon rises at sunset, opposite the Sun in our sky, and sets at dawn. This is Noon on the Moon, and if you had been graphing the apparent brightness of the Moon as a percent of illuminated area, you would be surprised to see a sudden jump in brightness. This is because the silicate minerals in the pulverized Moon rocks making up the surface (the regolith) have a tetrahedral crystalline structure that make them efficient retroreflectors, sending the light straight back to us.

The Moon has a reflectivity, or albedo, of about 0.12, meaning its surface generally reflects about 12% on the light that falls on it. That's about the same albedo as the gray of weathered asphalt pavement, although in contrast with a dark sky we perceive it as looking much brighter. The Moon is in the sky just as much in daytime as it is at night, but it is not as obvious against a bright blue background.

The Waning (shrinking) Gibbous phase brings the Moon back to half illuminated, as the sunset terminator begins to sweep along in a mirror to the waxing phases. The waning point of half illumination is Last Quarter, with the Moon rising near midnight, and setting near noon, once again 90° from the Sun in the sky. Then the Waning Crescent phase brings us back to astronomical New Moon, when the Moon is in the same direction as the Sun in our sky, and we see no illuminated surface - midnight on the Earthward face of the Moon.

These phases are caused by the changing angle of illumination of the Sun on the surface of the Moon, and have nothing to do with the shadow of the Earth. The dark area of the Moon is, in effect, the shadow of the Moon itself.

Let's look at this progression from space, above the north side of the plane of the Earth's orbit around the Sun, with the right side of the image corresponding to the direction of the Sun at the Vernal Equinox, at 9:57 AM PDT on March 20, 2014. The sizes of the Earth and the Moon have been exaggerated by a factor of 25. It's not obvious in this animation, but the Earth is rotating about seven times between each frame. The entire period between New Moon to New Moon is the Moon's Synodic Period, 29.53 days. This is why pure lunar calendars eventually get out of phase with the seasons - 12 months of 29.53 days each is only 354.36 days.

There is no permanently Dark Side of the Moon. A typical spot on the Moon's surface sees about 14 days of sunlight, followed by 14 days of night. Because the Moon's rotation is tidally locked to the Earth, it rotates once around its axis in the same amount of time it takes to revolve around the Earth, and thus we always see essentially the same face of its surface. The other side is the Far Side, a much better term for the side we don't see, and a comic we no longer see.

You can see the Earth's terminator moving as the apparent position of the Sun changes by about a degree each day, because the Earth is revolving around the Sun. As a result, the period for the Moon to return to the same position in our sky relative to the stars, its Siderial Period, is about 27.32 days.

Watching the orbital animation might make you wonder why we don't see a total lunar eclipse with every Full Moon, when the shadow of the Earth would extend in that direction. At New Moon, you might expect to see a solar eclipse, with the Moon blotting out the Sun. However, there is a third dimension, or depth into the image, that isn't apparent.

The orbital plane of the Earth around the Sun, corresponding to the circle around our sky that the Sun appears to follow, is called the Ecliptic, because that's where eclipses happen. The orbital plane of the Moon is tilted relative to the ecliptic by just over 5 degrees. The apparent angular size of the Moon is about half a degree, or 30 arcminutes. Therefore, the Moon can easily pass above or below the Earth's shadow or the visual disk of the Sun, also about 30 arcminutes across.

The points where the lunar orbital plane intersects with the ecliptic are called its nodes, and it is only when the Moon is near the nodes of its orbit at Full and New phase, so the line of the nodes points toward the Sun, that we have eclipses. That happens for 2014 in April and October.

Weather permitting, on the night of April 14/15, between about 9:53 PM and 03:38 AM PDT, we will experience a total lunar eclipse, with the deep umbral shadow of the Earth completely covering the Full Moon from about 12:06 AM to 1:24 AM. Two weeks later, with the New Moon of April 29, there will be a brief annular solar eclipse, visible from a small area of Antarctica.

In the early morning hours of October 8, there will be a second total lunar eclipse visible from Santa Barbara, between about 1:15 AM and 6:34 AM PDT. Totality will last from about 3:25 AM to 4:24 AM. Around two weeks later, on the afternoon of October 23, we will see a partial solar eclipse from about 2:06 PM to 4:37 PM PDT. About 45% of the Sun's disk will be covered by the Moon.

The Astronomical Unit is not planning any public observations of the two total lunar eclipses, since they are occurring in the wee hours when it is difficult to find an appropriate venue with parking and restroom facilities that is open. However, for the partial solar eclipse, we plan to have safely-filtered telescopes and other viewing devices set up at the Camino Real Marketplace in Goleta, so you can stand in the edges of the Moon's shadow.

References for a Cloudy Evening

The Moon images for this article were in part generated using NASA Goddard Space Flight Center's Scientific Visualization Studio. NASA.gov

NASA Eclipse Page

Santa Barbara Astronomical Unit


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