The Lunar Math Of The Moon’s Phases

Only in the night sky does a quarter equal a half and a half is full. It’s not new math — it’s lunar math. When we see a first quarter Moon, it looks like a half moon, so perhaps you’ve wondered why it’s called quarter. Like planets, the Moon emits no light but rather reflects sunlight as it orbits Earth every four weeks (more precisely, 29.53 days). When it’s between Earth and Sun at new Moon, we don’t see it as the Sun illuminates the side facing away from us.

Only in the night sky does a quarter equal a half and a half is full. It’s not new math — it’s lunar math. When we see a first quarter Moon, it looks like a half moon, so perhaps you’ve wondered why it’s called quarter.

Like planets, the Moon emits no light but rather reflects sunlight as it orbits Earth every four weeks (more precisely, 29.53 days). When it’s between Earth and Sun at new Moon, we don’t see it as the Sun illuminates the side facing away from us.

A day or so after new Moon, we begin seeing a slight sliver soon after sunset, called a waxing crescent — waxing because it gets more illuminated each night and crescent because of its appearance from our perspective.

In a week, when it has traveled a quarter of the way around Earth, its 1st quarter phase looks half-lighted to us.

Then for the next week as the Moon continues to wax, it appears more than half illuminated, but less than full — a phase called gibbous (Latin for hump).

After two weeks, the Moon has completed half its journey and is on the opposite side of Earth from the Sun which illuminates the entire half of the Moon facing us — called a full Moon.

Then for the next two weeks, the Moon become less illuminated each night, called a waning Moon. During the third week it is in its waning gibbous phase on its way to 3rd quarter (sometimes called last quarter) when it again appears half illuminated.

And during the last week of it sojourn, it is in its waning crescent phase (seen in the morning sky) until it again reaches new Moon and starts its next monthly cycle.

Regardless of how much of its surface we happen to be seeing on any given night (or day), half of the Moon, just like half of the Earth, is always illuminated — whichever half is facing the Sun.

• Sky Calendar.
* Mar. 21 Sun.: Saturn is at opposition — on the opposite side of Earth from the Sun — when it rises at sunset, is up all night, and sets at sunrise.
* 23 Tue.: The Moon is at 1st quarter.
* 24 Wed. evening: The Moon is to the lower right of Mars, and then to the planet’s lower left the next night.
* 28 Sun. all night: The Moon accompanies Saturn across the sky.
* 29 Mon. morning: The full Moon is called Lenten Moon, Sap Moon, Crow Moon, and Worm Moon.
* Apr. 3 Sat. morning: The gibbous Moon is less than two moonwidths above the star Antares in the south.
* 6 Tue.: The Moon is at 3rd quarter.
* 8 Thu. evening: Mercury, at its best for this appearance, is to the lower right of Venus low in the west at dusk.

• Naked-eye Planets. (The Sun, Moon, and planets rise in the east and set in the west due to Earth’s west-to-east rotation on its axis.) Evening: Saturn is low in the east with Mars high overhead; Venus is very low in the west after sunset. Morning: Saturn, low in the west, is now the only morning planet.

Stargazer appears every other week. Paul Derrick is an amateur astronomer who lives in Waco. Contact him at 918 N. 30th, Waco, 76707, (254) 753-6920 or paulderrickwaco@aol.com. See the Stargazer Web site at stargazerpaul.com