Observing the Moon, Part 1: Appearance and Phases

By Peter Forrester | January 27, 2019

I skipped over the Sun and the Moon when I wrote about the planet Venus. They are the only two natural objects in the sky brighter than Venus. I will deal with the Sun when we are getting close to a solar eclipse, as that is the only time it is safe to look at it (unless you saved eclipse glasses from the last one, and make sure there are no holes in them. Do not try to use any other type of glasses).

The Moon is our nearest neighbor in space (about 238,000 miles from Earth, on average), and it is the only place outside of Earth orbit where humans have visited (only during six U.S. Apollo missions between 1969 and 1972). It is also the only object, other than the Sun, which is close enough to appear as more than a point of light when seen by the unaided or naked eye. This makes it possibly the most interesting object for observation.

The Moon is very bright, with an apparent magnitude of -12.74 (this is all light from the Sun reflecting off its surface – keep that in mind for later). It is also a relatively large  satellite compared to its planet, the Earth. It is the fifth largest natural satellite in the Solar System (the other big ones all orbit gas giants). The diameter of the Moon is more than one quarter the size of the Earth’s.

Put this together, and you get that the Moon is easy to see, but that only half of it is lit at a time. One more piece to add in.

The Moon is “tidally locked” with respect to the Earth. This means that we always see the same side of the Moon from the surface of the Earth (technically, it wags from side to side slightly, a process known as “libration”, and so about 59% can be seen at different times in its orbit). The tidal locking is because the Moon’s rotation about its axis and its revolution around the Earth both take about the same amount of time, 29.5 days.

It is accurate to refer to the near side or far side of the Moon – but not astronomically correct to speak of a “light side” or “dark side”, unless you are strictly speaking from the perspective of someone on the Moon. The phases of the Moon occur as the lighted area of the Moon, that which faces the Sun, moves across the near side. We see the entire lighted portion during a Full Moon, and none during the New Moon.

The Moon’s orbit is tilted about 5 degrees from Earth’s orbit around the Sun. It is only when the orbits line up precisely that eclipses occur: a lunar eclipse during a Full Moon, and a solar eclipse during the New Moon. The solar eclipse is possible only because the apparent size of the Sun and Moon are about the same, about one half degree. Space probes have observed eclipses where this was not the case..

During a lunar eclipse, the Moon passes through the shadow of the Earth, and during the period of totality, the only sunlight falling on the Moon is that which is passing through the Earth’s atmosphere. The same bending of light rays that produces the reds of sunrises and sunsets also causes this light to turn red, as we just saw a few days ago on the eclipse on January 20th (if you were lucky enough to be able to see it, as I was not).

The entire phase cycle of the Moon takes 29 1/2 days. About 7 days after each New Moon there will be a First Quarter, followed by a Full Moon a week later. During this period it is said to be “waxing” or the lighted portion as seen from Earth is getting bigger. After the Full Moon it is “waning” or getting smaller until the next New Moon. When the phase is between a Quarter and Full Moon its shape is called “gibbous”. When it is between Quarter and New Moon, we call the shape a crescent.

One more detail about the appearance during phases, and this is dependent on which hemisphere you’re in. Here in Milton, in the Northern Hemisphere, the light moves from right to left. At first, we will see the dark portion of the phase on the left (the East) when the Moon is waxing. Later when it is waning, the right side will be dark. (This direction is opposite south of the equator, where the Moon and stars appear 180 degrees inverted or rotated).

I will cover more detailed observation of the Moon in a subsequent article, with topics such as the craters and seas of the Near side, and the names they are called by.

Happy Moon-watching!


References:

Wikipedia. (2019, January 25). Lunar phase. Retrieved from https://en.wikipedia.org/wiki/Lunar_phase.

Wikipedia. (2019, January 25). Moon. Retrieved from https://en.wikipedia.org/wiki/Moon.

Author: Peter Forrester

I have been interested in astronomy and stargazing for many years, and now delight to offer some of my learning to others through my weekly blog posts.

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