Why do we see different phases of the moon?
The Moon orbits Earth at the same speed it takes Earth to rotate around its axis, but the Earth orbits the sun, so no matter how the Moon itself looks, its visible shape is actually just determined by where the light from the Sun shines on it.
This is also why we see only one side of the Moon. Its geosynchronous orbit with Earth means that it is turning at the same rate that the Earth spins, which presents us with the illusion that it is not spinning at all.
Phases of the Moon
The Moon’s phases occur on a 29.5-day cycle.
- New
- Crescent
- First Quarter
- Gibbous
- Full
- Gibbous
- Third Quarter
- Crescent
Phases 1-4 (New, Crescent, First Quarter, Gibbous) are the ’waxing’ phases of the moon, during which it is visible in the afternoon/evening. During the waxing phases, the moon appears to get “fuller” and rises later and later each day.
Phases 5-8 (Full, Gibbous, Third Quarter, Crescent) are the ’waning’ phases of the moon, where the moon appears to be shrinking every night. Phase 5 is the full moon, during which the moon rises when the sun sets and sets when the sun rises, making it visible only in the night or early in the mornings. All of the waning-phase moons are visible in the late night or early mornings.
Rise and Set Times
The moon rises and sets in our local sky depending on its phase. During the full moon it rises at about 18:00 and sets at about 06:00, while during the new moon it rises at about 06:00 and sets at about 18:00. You can find the values between these by incrementing evenly.
What causes eclipses?
Solar Eclipses
Solar eclipses only occur during new moons, when the Moon’s 5 degree orbital tilt makes it cross paths with the ecliptic plane of the Earth’s orbit around the Sun. These locations are called nodes.
During a solar eclipse, the Moon blocks the view of the Sun— which, despite being vastly different in actual size, is about the same angular size as the moon when viewed from Earth— from Earth, but the light from the Sun makes a “halo” around the dark moon in the sky (since no light is shining on it). It’s like a new moon on steroids.
Total Solar Eclipse
A total solar eclipse occurs where the Moon’s full shadow touches the Earth.
Partial Solar Eclipse
A partial solar eclipse occurs where some of the moon’s shadow is touching that point on Earth, but not all of it. The shadow kind of goes outward in a cone shape.
Annular Solar Eclipse
If the Moon’s full shadow does not reach Earth, an annular eclipse occurs in the region directly behind it. This happens when the shadowy moon does not fully cover the sun, and yields a kind of “ring of fire” effect.
Lunar Eclipses
Lunar eclipses occur when the shadow of the Earth falls on the moon. This essentially just boils down to lunar eclipses being when there is a full moon and the Moon crosses an orbital node as described before.
During a lunar eclipse, the Earth is aligned between the Sun and the Moon like a sandwich. From here, the Sun casts Earth’s shadow onto the moon. This also means that a lunar eclipse will only happen when the Moon passes through the Earth’s shadow.
Total Lunar Eclipse
A total lunar eclipse occurs when the entire moon is fully passing through Earth’s shadow.
Partial Lunar Eclipse
A partial lunar eclipse occurs when part of the moon passes through the Earth’s shadow, but not all of it.
Penumbral Lunar Eclipse
A penumbral lunar eclipse occurs when the moon only passes through the partial shadow of the Earth— i.e., instead of the Earth’s shadow being fully cast onto the moon, it only blocks some of the light from directly reaching the moon’s surface.