Astro Bob: April's pink moon biggest of the year

"Astro" Bob King is a freelance writer for the News Tribune. Read more of his work at

Full moon moonrise Oct-6_2017 S2.jpg
A full and colorful moon rises over Lake Superior in Duluth, Minn. The Full Pink Moon occurs on Tuesday, April 7. The moon will appear fullest at 9:35 p.m. Central Time (10:35 p.m. Eastern, 7:35 p.m. Pacific). Bob King, for the News Tribune
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Named for the color of the early spring phlox flower April’s full moon falls on Tuesday the 7th. Pink seems appropriate for a full moon. It often appears pinkish-orange when it’s low in the sky around the time of moonrise. That’s because the moon’s light must travel a long path through the lower atmosphere where the air is thickest.

White sunlight and moonlight are composed of all colors of the rainbow. When they pass through Earth’s atmosphere, the air scatters the blue and violet part of sunlight across the sky to color it blue.

Recall that white light is composed of every color of the rainbow from violet to red. Air molecules — 98 percent of which are nitrogen and oxygen — scatter the violet, blue and green colors in moonlight in all directions, which in turn colors the sky blue. The leftover red, orange and yellow colors easily penetrate the air and zoom directly to your eyes. That’s the reason the moon looks so warmly colorful when it rises. Blue light has a shorter wavelength (tinier waves) and is more easily scattered. Red light has a longer wavelength (bigger waves) and resists scattering.

A moment ago I said that scattered moonlight turns the sky blue. It doesn’t look blue to our eyes because moonlight is much fainter than sunlight. But if you were to take a time exposure photo on a moonlit night you’d discover that a moonlit sky is as blue as the daylight one.


Dust and humidity can also affect the rising moon’s color but here’s a fun fact. Even if the air were perfectly transparent with zero dust or humidity the moon would still look pink-orange at moonrise. It’s all about the molecules. As the moon rises higher and higher its light passes through less and less of the densest air at the bottom of the atmosphere. Less air means less scattering. The result? The moon gradually whitens as it climbs up the sky. Indeed, a high moon appears virtually colorless.

The moon’s orbit around Earth is an ellipse with one end closer to the planet (perigee) and the other farther (apogee). A supermoon occurs when the full moon is close to perigee. Not to scale. Bob King, for the News Tribune

In 2020, the Pink Moon is officially the biggest full moon of the year. Especially large full moons are also called supermoons. Supermoons occur when the moon is at or near full phase at the same time it’s reaches perigee, the closest point to Earth in its orbit. The moon’s average distance from the Earth is 238,000 miles (382,900 km), but at perigee it’s about 30,000 miles closer compared to apogee, the most distant point in its orbit.

That’s significant! Not only is a full perigee full moon 14 percent larger it’s 30 percent brighter than the same moon at apogee.

The big question is — can you tell the difference in size with your eyes alone? Some people claim they can but I’ve tried and have never been certain. There is at least one way you can know for sure though. Take a picture with your smartphone.

The moon’s size from perigee to apogee varies by 14 percent. When the two different-sized moons are overlapped, the difference is obvious. Fred Espenak


Astronomers measure the apparent diameter of objects in the sky using degrees and fractions of degrees called arc-minutes and arc-seconds. Imagine a circle around the whole sky. That’s equal to 360°. One-three-hundred-sixtieth of that circle equals 1° or about two full-moon diameters. A single full moon measures about ½° across. One degree is equal 60 arc-minutes, written as 60′) so a full moon is about 30′ in diameter (31.2′ on average).

During the April 7th supermoon, the moon’s apparent diameter will be 33.8′ — about 2.5′ larger than normal. In contrast, the minimoon — the farthest full moon from Earth — occurs Oct. 30th when it will appear about 29.5′ across.

The moon from last night on my smart phone at max zoom. Bob King, for the News Tribune

The difference between the two extremes amounts to about 4 arc minutes. While we might struggle to see this with our eyes — especially since we can’t see both far and near moons simultaneously — we can record the difference with our smart phones. On April 7 point your phone at the moon and zoom in to maximum. Hold it steady, or buttress it against a door frame or the side of your car, and take several images to get a sharp one.

I would also recommend you manually adjust the exposure to tame the brightness of the moon. On an iPhone tap the moon’s image and a slider with a sun icon will appear. Slide slider downward to decrease the exposure and darken the moon. Click here for information on doing the same with an Android.

Six months from now on Oct. 30 do the same thing and then place both photos side by side or better, overlay them into a single image. The difference between supermoon and micromoon should be obvious.

I also came up with another way to see the difference between perigee and apogee moons using a simple homemade sighting device . Whichever way you chose, don’t miss seeing the extra-large moon at moonrise Tuesday. The color, its apparent flattening due to refraction by the atmosphere and the simple pleasure of experiencing Earth’s rotation by lunar proxy are enough reasons to set aside a half-hour for your own personal moon expedition.


Find a location with a good view of the eastern horizon. The moon will rise around sunset in the constellation Virgo not far from the bright star Spica. Find out specifically when the moon will rise for your location with this moonrise calculator . Clear skies!

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