The April full moon peaks today (April 11), reaching its fullest phase late at night, just after the bars close in most cities on the U.S. East Coast. But it will still appear full tonight to casual observers.
April's full moon is known as the Pink Moon, but don't expect it to look particularly pink. It is named after pink flowers called wild ground phlox, which bloom in early spring and become widespread throughout the U.S. and Canada this time of year. In fact, the April full moon is the first full moon of spring in the Northern Hemisphere (it's the first fall season full moon in the Southern Hemisphere).
Technically, the full phase — when the moon is on the exact opposite side of the Earth as the sun — occured at 2:08 a.m. EDT (0608 GMT). For skywatchers on the West Coast, the moon will reach peak fullness at 11:08 p.m. PDT on Monday (April 10). That said, the moon will appear full to casual observers everywhere from April 10 to April 12. [The Moon: 10 Surprising Facts]
For Northern Hemisphere observers along the East Coast, the sun and moon will rise and set nearly simultaneously. On April 10, the sun will rise over New York City at 6:25 a.m. local time, just 3 minutes after the moon dips below the horizon. That evening, the moon will rise at 7:03 p.m., nearly half an hour before sunset. On Tuesday (April 11), the moon will set at 6:51 a.m., following the sunrise by only 30 minutes. Thirty minutes after sunset, the moon will reappear at 8:02 p.m. — about 3 hours before its fullest phase. On Wednesday (April 12), the moon will be just past full and won't rise until 8:59 p.m. EDT.
Observers in Los Angeles will see the fullest lunar phase on Monday night, and as the city is farther south than New York, the moon will be somewhat higher in the sky over the course of the evening. That means the differences between moonset and sunrise are a bit different than New York, with moonrise at 7:04 p.m. and sunset at 7:21 p.m. – the amount of time the moon and sun share the sky is cut nearly in half. To find out when the moon will be out at your location, check out this moonrise and moonset calculator.
Moon phases explained
A full moon occurs every 29.5 days, when the side of the moon that faces the Earth is fully illuminated. The exact timing depends on where the moon is in its orbit around the Earth, which is why it doesn't always occur at night for viewers in different locations.
The phases of the moon occur because we see it from different perspectives as it revolves around the Earth. When the moon is 90 degrees to the left or right of the line connecting the Earth and sun, it's half-illuminated. Confusingly, this is called a quarter moon, but that's because it's a quarter of the way around its orbit.
The moon moves relatively quickly against the background stars; a careful observer might see it move east about a half a degree, or one moon diameter, per hour. Over the course of a 12-hour night, that's 6 degrees, which can take it right out of one constellation and into another. [How to Measure Distances in the Night Sky]
On the evening of Sunday (April 9), the moon will rise in the Virgo constellation, about 12 degrees east of Jupiter. By the time it sets Monday morning, Jupiter and the moon will be only 7 degrees apart. On Monday night, when the moon still looks full and rises, it will be within 2 degrees of Jupiter. The closest approach won't be visible to people in the U.S., but those in Europe and points east should be able to see it. U.S.-based skywatchers will have to be satisfied with seeing Jupiter rise just a few minutes before the center of the moon gets above the horizon, though over the course of the night the moon will pass near Spica and form a small, bright triangle with Jupiter.
When the moon is full it's easy to see some details – the lunar maria, for example. Those large, dark areas are lava plains, created by huge impacts that penetrated the crust. With a small telescope or binoculars one can see more details, but the full moon is so bright that a filter is needed to help bring them out. In fact, if looking with a larger telescope, it's sometimes harder to see details of the terrain because there are few shadows during the full moon.
What's a Pink Moon?
In many cultures, people named the full moons as they happened throughout the year; it was a way to keep time. Now we sometimes use nicknames derived from Native American myth or old European folk tales. According to the Old Farmer's Almanac, the April full moon is known as the Pink Moon because it marked the appearance of a flower called the moss pink, or phlox. That's likely a Native American tradition because phlox is common in North America and Siberia, as opposed to Europe. It is also called the Sprouting Grass Moon, the Egg Moon and the Fish Moon.
April full moons mark festivals and holidays in other parts of the world: in India, Hindus celebrate the birth of Hanuman in a festival called Hanuman Jayanti. The April full moon will also mark the beginning of Passover for the Jewish people. In Arabic-speaking Islamic traditions, the night of the full moon is called badr, for full, but the root of the word also has connotations of health and beauty, says Fadwa El Guindi in "By Noon Prayer: The Rhythm of Islam."
For Christians in England, April was known by the Old English name ēastre-monaþ, or "Easter month." The date of Easter is the first Sunday following the full moon that itself follows the vernal equinox, on March 21. This year Easter falls on April 16, a week after the full moon.
Editor's Note: If you capture an amazing photo of the Pink Moon and want to share it with Space.com for a story or gallery, please send images and comments in to managing editor Tariq Malik at firstname.lastname@example.org.