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 from it.
But, since the Moon orbits the Earth and the Earth orbits the Sun, the angles between these three bodies is constantly changing. But in predictable ways.
From our standpoint here on Earth, those changing angles are the cause of Moon Phases.
You might know four of the Moon’s phases:
- New Moon
- First Quarter
- Full Moon
- Last Quarter
These are known as the Moon’s “Major Phases“. But it has another four phases which happen between these major phases. These are the 8 phases:
- New Moon: The Moon’s un-illuminated side is facing the Earth. It is not visible.
- Waxing Crescent: The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of its disk that is illuminated is increasing.
- First Quarter: One half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon’s disk that is illuminated is still increasing.
- Waxing Gibbous: The Moon appears to be more than one-half but not fully illuminated by direct sunlight.
- Full Moon: The Moon’s illuminated side is facing the Earth and is fully illuminated by the Sun.
- Waning Gibbous: The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of its disk that is illuminated is decreasing. The fraction in shadow is on the opposite side to the Waxing Gibbous phase.
- Last Quarter: One half of the Moon is illuminated (the opposite half to First Quarter).
- Waning Crescent: The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of its disk that is illuminated is still decreasing. Happens on the opposite side to Waxing Crescent.
And then the cycle repeats, stating with another New Moon.
Did you know that a Full moon occurs at an instant in time – down 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 Full Moon (also around 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 time of the exact Full Moon.
On the night before and the night after, it will be 97-99% illuminated, although most people won’t notice the difference.
Even two days from Full Moon, the Moon’s disk is 93-97% illuminated and a casual observer will be hard pressed to notice that the Moon is not full.
The Lunar Month
A complete cycle of Moon phases takes an average of 29.5 days.
The “age” of the moon is the time in days counted from the last New Moon. That’s why some people describe the moon as being 3 days old, for instance.
Each complete cycle of phases is called a “lunation” – a lunar month, during which time the Moon completely circles the Earth in its orbit.
Because the moon phase calendar doesn’t match a calendar month, nor is it an exact multiple of a number of Earth days, the times of moon-rise and moon-set 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 the Metonic Cycle.
Lunar Eclipses
A lunar eclipse only happens during a Full Moon. The Sun, Earth and Moon are all in a line in space. The Sun is fully illuminating the Moon as we see it from Earth.
What’s special in these circumstances is that the Earth sits directly between the Sun and Moon and blocks the light falling on the Moon.
Lunar Eclipse is the term given to that time when the Moon is passing through the Earth’s shadow. Sunlight is prevented from falling directly on the Moon.
So why can we still see the moon during an eclipse?
The Earth’s atmosphere sunlight onto the Moon in a kind of ghostly glow. Our atmosphere absorbs all the blue light so only the red light gets refracted.
That’s why the Moon looks red during a total lunar eclipse.
So why isn’t there a total eclipse every month during Full Moon?
The Moon’s orbit is inclined to Earth’s equator by about 5 degrees. That’s just enough that the Moon doesn’t always pass through Earth’s shadow at Full Moon. It skirts either above or below Earth’s shadow.
On average, though, there is a lunar eclipse every 18-24 months. Not as uncommon as you might have thought!