Phases of the Moon Introduction

A recent survey of the graduating students from the Harvard School of Law discovered that 90% of the students could not describe the causes of the Moon's phases. Ideas about the Moon or Earth shadow abounded, as did a host of different celestial alignments which were Ptolemaic. While this is a relatively short page, I consider it to be exceedingly important for the Astronomy student to study. I will introduce the basic principles, and then move on to the subtleties that make a study of the lunar phases more interesting and complete.

This page, and the one that follows took a long time to create. With videos available, I am including a few here now, to give you a general picture before you do further reading.

NASA Video - 3 min 35 sec video ... very simple with one clear error, but it covers phases, tides, gravity, surface stuff, etc.

Lunar Phases - 2 min 24 sec video ... even more simple with a HUGE error ... can you find it? Hint (look at the images of the Moon) good at names of phases and the actual appearance of the phases!

In brief, the phases of the Moon are caused by the position of the Moon in its orbit around the Earth, and the amount of the sunlit half of the Moon that we see during that night. The image below depicts the Moon in its orbit around Earth. For simplicity, we will learn that the Moon requires 28 days to completely orbit Earth. Since the Earth spins completely in 24 hours, the Moon will move relatively little during those hours relative to its 28 day orbit. Thus, everyone on Earth will see the same phase during a given night. At any given time, half of the Moon will be sunlit, and the other half will be in shadow, as evidenced by the inner circle of moons that represent the position of the Moon in its orbit. Note too, that half of the Earth is also in sunlight, and half is in shadow.

This diagram has the days listed, so students can see that 28 days are required for the moon to complete its orbit, and also so students can identify a phase with a particular day position. Notice too that the Moon orbits in a counterclockwise direction. Every planet orbits the sun in a counterclockwise direction, and almost every moon too. From this page, when the Moon is at Conjunction, or Day 0, half of the Moon is sunlit, but humans living on Earth do not see any of that sunlit half. We only see the half that is completely in shadow. This phase is called New Moon, and is represented by the outer ring of moon images.



When the Moon is at Opposition (opposite the Sun), 14 days later, half of the Moon is still in sunlight, but now people on Earth see that entire sunlit half. This phase is called Full Moon.

People in Russia do not see a New Moon when Americans see a Full Moon. Look at the image again. When the Moon is at Opposition, the Full Moon is visible during the night, as evidenced by the shadowing of Earth. When Americans see a Full Moon, it is during their night. Opposite America, Russia is experiencing Noon, but the Moon is not up in their sky, since it is opposite of their place on Earth. Only when the Earth continues to rotate into view of the Moon while it is at Opposition will the Russians see a Full Moon ... some 12 hours after we see it.

Also, the Full Moon looks like it is completely in the shadow of the Earth. The drawing is deceptive for in reality, the Moon is slightly above or below the Earth-Sun line. The orbit of the Moon is tilted relative to the orbital plane of the Earth by about 5 degrees, so that it is not in the shadow of the Earth ... UNLESS the timing of Full Moon and Earth-Sun-Moon lineup on the same plane occur simultaneously. Then we would experience an Eclipse.

When the Moon is at Day 7 or Day 21, one half of the Moon is still lit by the Sun, but people on Earth only see one half of that sunlit half. The other half of the Moon that is in the sky is in shadow. We call these phases First and Last Quarters respectively. If the right half is lit, then the phase is First Quarter. When the left half is lit, then the phase is Last Quarter. In fact, whenever the most right parts of the Moon are lit, it is in a Waxing Phase, meaning "growing larger" relative to the amount of lit Moon we see. If the most left parts of the Moon are lit, it is in a Waning Phase, meaning "diminishing" relative to the amount of lit Moon we see. Crescent and Gibbous phases are the result of being someplace between Quarter or New /Full phases.

The small image found to your left shows nice photographic images of the phases. By clicking on the image, you can see a much larger version (may take a long time to load). As you look at these images, what do you notice?










This long gap here is to give you time to think about my question. I would normally ask my class students to raise their hands, but it is hard to do unless you have a webcamera and are standing away from the lens enough for me to see your hand. Also, I typically ask a question and the kids just sit there. So, I might patiently wait for them to work up enough courage to actually guess. What you should notice is that the same features are always present in our view. This is because the same side of the Moon is always pointed at the Earth.

Phase calculator

Blue Moon

More, complete, and better explanation about the*** Moon's Phases *** exists where you can find the "truth" about the length of a lunar cycle, or return to Moon Introduction, or the Syllabus.

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