Did you know that the Moon is the second brightest object in the sky, after the Sun?
We see the Moon because of the sunlight reflected off it.
But, since the Moon orbits the Earth and the Earth orbits the Sun, the angles between these three bodies are constantly changing. But in predictable ways.
From our point of view here on Earth, those changing angles are the cause of the lunar phases.
You may know four of the phases of the Moon:
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New Moon
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First quarter
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full moon
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Last room
These are known as those of the Moon “Main phasesBut it has four other phases that happen between these main phases. These are the 8 phases:
- New Moon: The unlit side of the Moon faces Earth. It is not visible.
- Crescent moon: The Moon appears to be partially illuminated by direct sunlight, but less than half. The fraction of your disk that is illuminated is increasing.
- First quarter: Half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon’s disk that is illuminated continues to increase.
- Waning moon: The Moon appears to be more than half full, but it is not fully illuminated by direct sunlight.
- Full moon: The lighted side of the Moon faces the Earth and is fully illuminated by the Sun.
- Waning gibbous: The Moon appears to be more than half full, but it is not fully illuminated by direct sunlight. The fraction of your disk that is illuminated is decreasing. The fraction in the shadow is on the opposite side to the waxing gibbous phase.
- Last room: Half of the Moon is illuminated (the half opposite the First Quarter).
- Waning crescent: The Moon appears to be partially illuminated by direct sunlight, but less than half. The fraction of your disk that is illuminated keeps decreasing. It occurs on the opposite side of Waxing Crescent.
And then the cycle repeats itself, indicating with another New Moon.
Did you know that the full moon occurs in an instant in time, up to the second?
So why does the moon appear to be full for several days in a row?
Because the percentage of the Moon’s disk that is illuminated changes very slowly around the time of the Full Moon (also around the New Moon, but the Moon is not visible at all then).
The Moon will appear to be fully illuminated only on the night closest to the exact time of the Full Moon.
The night before and the night after it will be 97-99% lit, although most people won’t notice the difference.
Even two days from the Full Moon, the Moon’s disk is 93-97% illuminated and a casual observer will have a hard time noticing that the Moon is not full.
Lunar month
A complete cycle of lunar phases takes an average of 29.5 days.
The “age“of the moon is the time in days from the last New Moon. That is why some people describe the moon as 3 days, for example.
Each complete cycle of phases is called “lunation“- a lunar month, during which the Moon rotates completely around the Earth in its orbit.
Because the lunar month does not coincide with a calendar month, nor is it an exact multiple of several Earth days, the moonrise and moonset times and the times of the phases change as the days and months go by.
The Moon only repeats the same phase on the same date (just under) every 19 years! This is known as Metonic cycle.
Lunar eclipses
A lunar eclipse only occurs during the full moon. The Sun, Earth, and Moon are all aligned in space. The Sun is fully illuminating the Moon as we see it from Earth.
What is special in these circumstances is that the Earth is directly between the Sun and the Moon and blocks the light that falls on the Moon.
Lunar eclipse is the term that is given to the moment when the Moon passes through the shadow of the Earth. Sunlight is prevented from falling directly on the Moon.
So why can we still see the moon during an eclipse?
Earth’s atmosphere is refracted.[1] the sunlight on the moon in a kind of ghostly glow. Our atmosphere absorbs all blue light, so only red light is refracted.
This is why the Moon looks red during a total lunar eclipse.
So why isn’t there a total eclipse every month during the Full Moon?
The Moon’s orbit is tilted relative to the Earth’s equator by about 5 degrees. That’s enough so that the Moon doesn’t always pass through the Earth’s shadow at Full Moon. It borders above or below the Earth’s shadow.
However, on average, there is a lunar eclipse every 18-24 months. It is not as uncommon as you might have thought!
[1] Refraction is a term that describes the curvature of light. We see that this happens when light moves from one medium to another, such as when it passes from air to water, or passes from vacuum (space) to air (our atmosphere).
Different wavelengths (colors) of light are refracted in different amounts. This is why you see a rainbow of colors when you shine white light through a prism. It is also the reason why you saw real rainbows. Water droplets in the atmosphere refract sunlight and break it into its constituent colors.