When the moon is full tonight, Earth’s lunar companion becomes a “blue” moon — in a floral, seasonal way.

Popular folklore holds that the second full moon in a given month is a blue moon, but tonight’s blue moon has another definition. It is the third of four full moons in a given season. In most seasons, there are just three full moons.

But this spring, there are four full moons between the vernal equinox on March 20 and the summer solstice on June 21. The third of the four full moons gets the blue designation — as a kind of placeholder — so named moons like the harvest moon, the snow moon and the wolf moon don’t become out of sync with their usual companion month, according to astronomer Geoff Chester, of the U.S. Naval Observatory.

The seasonal blue moons occur every two to three years on average. The next one is set for Aug. 22, 2021.

Chester said this seasonal blue moon — which is the original definition of a blue moon — was first dreamed up by the Maine Farmer’s Almanac in the early 20th century.

Thus, May’s usual flower moon becomes a blue flower moon, if we conform to the Almanac.

Sky conditions should allow for good viewing in most of the eastern U.S., but clouds will obscure the night sky in much of the western two-thirds of the nation.

Brian Murphy, a professor of physics and astronomy at Butler University in Indiana, notes that when this full moon first rises, it might have an orange tint as it starts to ascend the eastern horizon.

While the sun is setting in the western sky, the full moon will be rising in the eastern sky, then the atmosphere scatters blue light molecules around and we can see the red wavelengths, Murphy said. “That’s why you see the sun or the moon — when rising or setting — taking on an orange hue,” he said. “When the sun or the moon go to their higher places in the sky, they take on their normal colors.”

For much of tonight, it will appear as a normal full moon. “There is not going to be anything odd about it,” Murphy said. “There won’t be color, as the moon will be its typical grayish white.”

If you can, find a clear eastern horizon, such as a beach. Look to the east-southeast to see the full moon rising at the horizon. By 9 p.m., the moon will be several degrees above the southeastern horizon.

There will be more than one reason to look up into the evening sky Saturday night; you might be able to spot the International Space Station. (Check NASA’s Spot the Station website — spotthestation.nasa.gov — for times at other locations). It will look similar to an airplane crossing the sky, but it will be a moving bright dot — lit by the sun. It will not have the telltale blinking lights of a jetliner. Look just above the horizon in the northwest sky. The space station should be visible for about six minutes before setting in the eastern part of the northern sky.

The space station will also make a lengthy pass over Washington, D.C., on Friday night starting at 10:17 p.m., rising in the west-southwest sky and setting six minutes later to the northeast.

Chester notes that Saturday is the 50th anniversary of the launch of Apollo 10 from Cape Canaveral, Florida. With astronauts Thomas P. Stafford, John W. Young and Eugene Cernan, the Apollo 10 mission conducted NASA’s dress rehearsal for the lunar landing of Apollo 11 in July 1969. They tested in lunar orbit how to rendezvous and dock the command module Charlie Brown and the lunar module Snoopy.

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