I’d like to think that I have a good awareness of the phases of the Moon and the basic cause of them (the Sun lighting up one side of the Moon and the way the Moon moves around the Earth).
However, helping my son with his homework, I was surprised to discover that I did not know the answer to some basic questions about the appearance of the Moon from the Earth, for example: Why is the Moon sometimes visible during the day? Is the Moon always visible at night? Is there a relationship between the phases of the Moon and when and where it is in the sky?
In other words, I found that I knew a good deal about the causes of the phenomenon – the external view – but very little about the result of it as an observer on Earth – which is, after all, the only view most of us will ever get. There are many great sources of information on the subject on the Internet, but they tend to be long and relatively complex and to focus largely on the overall picture, rather than the simple question of how the Moon looks from Earth.
This post provides a simple explanation of just a few key points about the position and appearance of the Moon that anyone should be able to verify for themselves, by looking into the sky over the course of a few days. For clarity, I will ignore the many complex details of the situation, precise angles, distances, times etc.; I will also only talk about the appearance from the point of view of the Northern Hemisphere. Please consult any of the many good online sources for more detail!
THE MOON RISES AND SETS LIKE THE SUN
To an observer on Earth, the Moon appears to rise in the East and set in the West, just like the Sun.
THE MOON APPEARS SLIGHTLY FURTHER EAST EACH DAY
Although the Moon rises and sets like the Sun, it moves across the sky slightly slower than the Sun does.
Suppose that we take a snapshot of the sky at midnight, on a night when the Moon is at its peak. The following night at midnight the Moon will appear to be further to the East than it was as the previous night.
If we continue the experiment for several days then the Moon will eventually be out of view at the time we take our snapshot, because it will not yet have risen. The Moon will then not be visible in the sky at midnight for a couple of weeks, until it eventually reappears over in the West. After a lunar month, the Moon will have returned to roughly the same position in the sky at midnight as it was when the first snapshot was taken.
If we repeat the experiment by taking a snapshot at noon each day, then the same pattern will be seen: each day, the moon is further to the East than it was the previous day, until it is no longer visible in the sky at noon.
This means that the for some of the month, the Moon is only visible in the daytime; for part of the month it is only visible during the night; and there are periods in between when it is visible in the late night and early morning; and when it is visible in the late evening and early night.
THE PHASE OF THE MOON DEPENDS ON THE POSITION OF THE MOON RELATIVE TO THE SUN
The phases of the Moon are the change from the full Moon, to the half Moon, to crescent Moon and gradually back again during the course of the lunar month.
The phases of the Moon are directly connected to the position of the Moon in the sky in relation to the Sun.
Suppose that we look at the sky at Noon. If the Moon is high up in the sky, close to the Sun, it will appear as a thin crescent. (Of course if it too close to the Sun, then it won’t be visible at all).
If the Moon is further over to the East or West, then it will appear as a larger crescent.
This also means that a crescent Moon can only be seen during the day, or in the early morning or early evening; the Moon cannot be a crescent shape if it is visible in the middle of the night.
The position of the Moon relative to the Sun also determines the direction of the crescent shape: if the Moon appears to the East of the Sun, then the Crescent will be on the right side of the Moon. Whereas if the Moon is to the West of the Sun, the crescent will be on the left side.
Next, suppose we look at the sky in the evening,when the Sun has just gone below the horizon in the West. If the Moon is over in the West then it will be a narrow crescent. Whereas if it is over to the East, it will be a ‘Gibbous’ moon – where the Moon is nearly full, but with a crescent shadow to the left side.
In the evening sky, the lighted side of the Moon will always be on the right side.
Looking at the sky in the morning, if the Moon is in the East it will be a narrow crescent, whereas it will be a Gibbous Moon if it appears in the West.
In the morning sky, the lighted side of the Moon will always be on the left side.
A full moon occurs when the Sun and Moon are on opposite sides of the Earth, so a full moon is only visible in the depths of the night.