After the longest day of the year Thursday, Wyoming stargazers will see something they haven’t for a generation when time will seem to stop and a huge full moon will hang still in the night sky Friday.
With the summer solstice comes the beginning of the “lunistice,” or lunar standstill, a major celestial event that only happens once every 18.5 years or so.
The full moon lunistice will be at its lowest point Friday, then reach its high point in six months.
That means there will be plenty to see after the summer solstice. The moon won’t rise very high Friday, but it’ll be big and bright.
“We're going to get lunar standstills separated by six months,” said Max Gilbraith, the planetarium coordinator for the University of Wyoming Physics and Astronomy Department. “The moon will get to its maximum extents in the sky, so it’s a pretty neat thing.”
What A Lunistice?
While we experience two solstices every year, a lunistice only happens about every two decades. Gilbraith said the reason for the disparity is how the moon moves through space.
“It's due to the misalignment between the lunar orbit around the Earth, which is tilted around 5 degrees, and the Earth orbits around the sun, which our equator or our North Pole, which is 23.5 degrees relative,” he said. “If you process that around, you get this 18.6-year cycle.”
The cycle of movement from the moon’s tilted orbit also dictates the frequency and location of lunar and solar eclipses.
That would explain why Wyoming got a front-row seat for the 2017 full eclipse, but mostly missed out on a full eclipse earlier this year.
“The cycle affects how long the moon is in the night sky and where it's rising and setting points will be,” Gilbraith said. “So, the lunar standstill is when the moon reaches southernmost and its northernmost point of rising and setting as seen from Earth.”
Gilbraith explained that just like a solstice occurs every six months, a lunistice goes through a similar six-month cycle.
That means Wyomingites can expect to see both extremes before the end of 2024.
“The moon will appear as far south as we can ever see it from our location on June 21,” he said. “The opposite happens this December, when our full moon will be as far north as we can see it.”
Strawberry Supermoon
The first full moon of the summer is called the strawberry moon. When the moon rises shortly before sunset Friday, the strawberry moon will appear bigger and brighter than normal.
The moon doesn’t rise high in the sky during the summer and stays low throughout the night. Gilbraith said that could create a super optical illusion that evening.
“Our brains play a trick on us whenever the moon is so bright,” he said. “When a full moon is on the horizon, our brain basically makes it look bigger than it is optically, because it's so bright. The June 21 full moon will appear almost like a supermoon when it rises.”
The moon could also be colored a deep red as it emerges on the horizon, which is how it got the nickname “strawberry moon.”
But it won’t get higher than 20 degrees in the sky thanks to the lunistice cycle.
Gilbraith said the moon’s appearance will be “a little disorienting” since it’ll be staying so low in the sky that night. In fact, the moon will rise and remain so close to the horizon, it’ll outrace the sun.
“It's a short, full moon rise through the night,” he said. “It will kind of beat the sun, because it has a smaller arc around the sky than the sun does right now, because it's so far south and it's lunar standstill.”
Looking For The Lunistice
Sunset will happen at 8:40 p.m. and thee lunistice begins 45 minutes earlier when the moon rises at 7:55 p.m.
The strawberry moon will stay on a low arc throughout the night, and set at 4:36 a.m. Sunrise Saturday is at 5:30 a.m.
The lunistice is especially remarkable because the full moon is happening on the night of the summer solstice. It’s the first time that’s happened since 1985.
Since the lunistice has its own six-month cycle, Gilbraith recommends everyone put Dec. 15, 2024, in their calendars now. That’s when “the cold moon” will reach its northern maximum in the night sky.
“It'll be 76 degrees high above the horizon that night,” he said. “It will be almost overhead, way higher than the sun ever gets as seen from most of Wyoming. With some fresh snow on the ground and the moon shining straight down overhead, that cold moon will be a really beautiful sight.”
Gilbraith also said that the moon will be in conjunction with Jupiter the night of Dec. 15. The two celestial bodies will appear as one, which is another unique orbital phenomenon to experience.
In the meantime, the National Weather Service is calling for mostly clear skies across Wyoming for the summer solstice, and Cowboy State Daily meteorologist Don Day says it should be "60%" clear by sunset.
There shouldn't be too much blocking the view of the lunistice.
