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 first quarter phase looks half-lit 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.

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 becomes less illuminated each night, which is why it’s called a waning moon. During the third week, it is in its waning gibbous phase on its way to third 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.

*  Next two weeks: Average sunrise: 7:24 a.m.; average sunset: 7:45 p.m.

Today is the spring (vernal) equinox, the northern hemisphere’s first day of spring when day and night are each about 12 hours long.

Tonight a crescent moon grazes the Pleiades star cluster, a sight best seen though binoculars; the reddish star to their upper left is Aldebaran, the “red eye” of Taurus the bull.

Tomorrow Saturn is at opposition on the opposite side of Earth from the sun; it rises at sunset, is up all night, and sets at sunrise. The moon is at first quarter Tuesday.

On Wednesday evening it is to the lower right of Mars, and then to the planet’s lower left the next night.

The night of March 28, the moon accompanies Saturn across the sky.

The March 29 full moon is called Lenten Moon, Sap Moon, Crow Moon and Worm Moon.

*  Naked-eye planets: (The sun, moon and planets rise in the east and set in the west because of Earth’s west-to-east rotation.)

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 Saturday. Paul Derrick is an amateur astronomer who lives in Waco.