Friday evening (July 23), the full moon will rise around sunset. Few may see it. So many areas in the U.S. and Canada have been plagued with smoke from wildfires this month I wouldn't be surprised if it took 15 minutes for the moon to show. I arrived at that number after watching a smoky sunrise a week ago, when I waited 12 minutes for the crimson ball to finally appear.

A kayaker cuts across the sun's glitter path on July 17, 2021. Even a half-hour after sunrise the sun still glowed deep orange. (Bob King)
A kayaker cuts across the sun's glitter path on July 17, 2021. Even a half-hour after sunrise the sun still glowed deep orange. (Bob King)

I try to see every full moon. I like the company of other moongazers which is why I often watch from a public beach on the shore of Lake Superior, my favorite locale. The shared enthusiasm for a big moon over big water makes my heart happy especially when parents bring their children along.

The moon will be full on Friday, July 23 and appear in the constellation Capricornus to the right of Saturn and Jupiter. (Stellarium)
The moon will be full on Friday, July 23 and appear in the constellation Capricornus to the right of Saturn and Jupiter. (Stellarium)

July's full moon is called the Buck Moon because antlers first start to grow on the heads of buck deer this time of year. It's also called the Berry Moon and Thunder Moon. I can vouch for the former because now's the peak of wild raspberry season (blueberries, too). I stumbled onto acres raspberry bushes two days ago and couldn't get the juicy drupelets into my mouth fast enough.

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When the moon is full, you could say it's at the end of its rope. The angle it makes to the sun is 180°, as wide as it gets. After full moon, that angle decreases night by night until it reaches 90°, and we see a half-moon. A week after that, when the moon lines up almost directly in front of the sun, the angle is near 0°, and it's new moon. Phases are caused by the moon's changing angle to the sun as it revolves around the Earth. This is nicely illustrated in the accompanying diagram (below).

The moon never stops moving. And since full moon phase is defined by the angle the moon makes to the sun, it lasts only a moment before the moon once again starts to wane. To the casual glance, however, the moon can appear full for a couple days around the official full moon date. A small telescope will quickly reveal the truth.

Full moon dates on calendars vary depending on which time zone they represent. The September full moon occurs at 7:55 p.m. EDT on the 20th or 12:55 a.m. on the 21st if you live in Central Europe. (Bob King)
Full moon dates on calendars vary depending on which time zone they represent. The September full moon occurs at 7:55 p.m. EDT on the 20th or 12:55 a.m. on the 21st if you live in Central Europe. (Bob King)

Sometimes you'll notice that the date of full moon varies a day from calendar to calendar. That's because one calendar might use Universal or Greenwich Time and another Eastern Daylight Time. For example, this week's full moon occurs at 10:36 p.m. Eastern Daylight Time July 23, equal to 3:36 a.m. July 24th Greenwich Time. You can check the exact times of the four primary lunar phases here.

To see the complete dayside of the moon (one full hemisphere), the moon has to lie exactly opposite the sun. That only occurs at full moon. At other times, only portions of the dayside are visible — the rest is hidden from view. (Bob King)
To see the complete dayside of the moon (one full hemisphere), the moon has to lie exactly opposite the sun. That only occurs at full moon. At other times, only portions of the dayside are visible — the rest is hidden from view. (Bob King)

Although it seems counterintuitive, half of the moon is always in sunlight and half in darkness, exactly like the Earth. The sunlit side of the moon and Earth is called the dayside. The only time we can see the entire dayside of the moon is at full moon phase. Only then is the moon positioned exactly opposite the sun from Earth. When we turn to face it, the sun shines "over our shoulders" and fully illuminates its face.

As the moon orbits the Earth, one hemisphere is always in sunlight. We see phases as the moon's angle to the sun changes throughout its orbit. (Orion 8 / CC BY-SA 3.0)
As the moon orbits the Earth, one hemisphere is always in sunlight. We see phases as the moon's angle to the sun changes throughout its orbit. (Orion 8 / CC BY-SA 3.0)

When only half the moon is visible at first quarter phase, we see half of the daylight hemisphere. The other half faces away from us on the moon's far side. At new moon we face the moon squarely again, but now it's between us and the sun, so we only see its nightside. The daylit side faces back toward the sun and would look like a full moon if we could just get back there to see it!

Wishing you a glorious moonrise Friday night the 23rd. Make sure you find a spot with a good view to the southeast. Click here to find out when it rises for your location.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.