Tonight, observers will see a special type of full moon that comes only once a year: the minimoon, or a full moon that appears slightly smaller than usual.
On June 9, at 9:09 a.m. EDT (1309 GMT), the minimoon will reach its fullest phase. At this time, the natural satellite will be below the horizon for observers in the continental U.S., but early risers in Hawaii and parts of Alaska will have a chance to see the minimoon at its fullest, weather permitting. (To figure out if and when you'll be able to see the tiny full moon, check out this moonrise and moonset calculator.)
Saturn joins tonight's moon show, too! Find out how to see it here:
Full Strawberry 'Minimoon' Shines with Saturn Tonight!
The peak full moon will be above the horizon in most of Asia and all of Australia. But skywatchers in Europe, along with most viewers in the U.S., will have to catch a glimpse of the almost-full moon on the evenings before and after the peak. To the casual observer, the moon will appear just as full between Thursday and Saturday. [The Moon: 10 Surprising Lunar Facts]
What's a minimoon?
A minimoon refers to a full moon that is at or near apogee, the point in the satellite's orbit where it is farthest from Earth. This is essentially the opposite of a supermoon, which refers to a full moon that occurs at perigee, or the point where the moon is closest to Earth. Minimoons look up to 14 percent smaller than supermoons and are slightly less luminous than regular full moons.
Minimoons and supermoons happen because the moon's orbit isn't perfectly circular; it's a tiny bit elliptical, with the Earth at one focus of the ellipse and the moon at the other. On average, perigee is about 225,700 miles (363,000 kilometers) from Earth and apogee is 252,000 miles (405,500 km), according to NASA.
A line drawn through both foci of this ellipse would be the ellipse's major axis, called the line of apsides by astronomers. The moon's line of apsides rotates relative to the stars once about every 8.85 years. That means the apogee and perigee won't always line up perfectly with a full (or new) moon. Sometimes, the perigee of the moon coincides with the moon's first quarter, for example, and that's why supermoons and minimoons don't occur every month.
While minimoons and supermoons have slightly different apparent sizes than normal moons, those differences are probably too small for amateur observers to notice, Ernie Wright, lead visualizer at NASA's Scientific Visualization Studio, told Space.com.
"Ancient peoples were acutely aware of many things about the sky, certainly more than the average person is now," Wright said. "But I don't believe anyone took note of the changing apparent size of the moon.
"The difference between the largest and smallest full moon is only 4 arcminutes, near the limit of what the naked eye can detect," he added.
That's less than a sixth of the average diameter of the moon in the sky, and one would have to compare it side-by-side to a moon that appeared several months earlier to really notice the size difference.
When to see apogee
The moment of apogee occurs on June 8 at 6:22 p.m. EDT (2222 GMT), about 14 hours and 47 minutes before the moon is officially full. The satellite's distance from Earth during this apogee will be 252,525 miles (406,401 km). As with the full moon the following morning, Thursday's apogee happens before the moon rises over much of the world.
Since the moon doesn't rise until 7:34 p.m. local time in New York City, observers there and along the rest of the East Coast won't quite catch the full moon at its farthest. On the other hand, if you're in Reykjavik, Iceland, you'll see the moon reach apogee at 11:22 p.m., in the constellation Ophiuchus. Observers in London or Paris will see the moon reach apogee much higher in the sky (at about 22 degrees above the horizon in the latter case). Parisians will see the moment of apogee at 12:22 a.m.
Besides reaching apogee, the full moon will also appear near the planet Saturn (which reaches opposition on June 15). The ringed planet will be just on the south side of the moon, rising only a few degrees to the right of the moon in the Northern Hemisphere. Saturn is bright, but the nearby moon will wash out the ringed planet somewhat. [A Planet Skywatching Guide for 2017: When, Where & How to See the Planets]
A minimoon with many names
Among European colonists of the Americas, the June full moon was known as the Rose Moon. Celtic-speaking peoples called it the Moon of Horses.
Some native peoples in the northeastern U.S. called this moon the Strawberry Moon, as June is when wild strawberries native to the Americas ripen in that region. That name was hardly universal, though (and the strawberry-ripening connection would vary, because the plant ripens earlier or later depending on location).
Names among Native Americans for the full moons differed according to the local culture and environment; the Cherokees called the full moon of June the Green Corn Moon, since that was the growing season, and the Diné (Navajo) called it a Planting Moon. The Tlingit peoples of the Pacific Northwest called the June full moon the Birth Moon, according to "Tlingit Moon & Tide," a teacher's resource for Native children published by the University of Alaska.
For Muslims, the June full moon occurs on 15 Ramadan, the month of religious fasting.
And while Northern Hemisphere cultures associated summer full moons with warm weather and abundance, in the Southern Hemisphere it was different; in New Zealand, the Māori lunar calendar called the first month of the year — which starts with the new moon of May and ends with the new moon in June — "Pipiri." The Māori people described this moon with the phrase, "All things on Earth are contracted because of the cold," according to the "Teara Encyclopedia of New Zealand." The night of the full moon was named Rākau-nui, accompanied by the (translated) phrase, "The moon is filled out. Produce from the sea is the staple food."
Whether you're enjoying the warm weather in the Northern Hemisphere or gearing up for winter in the Southern Hemisphere, the minimoon is worth checking out. Even if this moon is below your horizon during its peak fullness on Friday or its apogee about 15 hours before that, the satellite will still look full and (sort of) small before and after those times. You won't have another chance to see a miniature full moon until July 27, 2018.
Editor's Note: If you capture an amazing photo of the little Strawberry 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 email@example.com.