The first flowers made their appearance in my neighborhood this past week. My premiere bloom was a single hepatica spotted along the hiking trail. A few days later, a sweet coltsfoot blossom caught my eye. April's full moon, which occurs Monday night the 26th, is named for an eastern phlox called moss pink that flowers this time of year. The name seems appropriate if only for the color, as the moon often glows a beautiful shade of pinkish-orange at rise and set.

During April, the moon's distance from Earth varies by 26,464 miles (42,590 km) from perigee to apogee due to its elliptical orbit. (NASA)
During April, the moon's distance from Earth varies by 26,464 miles (42,590 km) from perigee to apogee due to its elliptical orbit. (NASA)

The Pink Moon will also be the second closest supermoon of the year. A supermoon is a full moon that coincides with the time the moon is closest to the Earth. Being closer, it looks both bigger and brighter. The moon's distance varies because it revolves the Earth in an ellipse with one end of its orbit closer to the planet than the other. When nearest, the moon is at perigee. When farthest, it's at apogee.

The term supermoon is a modern invention coined by astrologer Richard Nolle in 1979. He defined it as "a new or full moon which occurs with the moon at or near (within 90 percent of) its closest approach to Earth in a given orbit". There's no particular reason to pick 90 percent, but he did, and now the concept has become a part of modern culture.

It would be more scientific to describe a close full moon as a perigee syzygy (SIH-zeh-gee). Say that quickly 10 times in a row. A syzygy is an alignment of three celestial bodies, in this case the moon, Earth and sun. Because the new moon isn't generally visible, supermoon is only used for the full moon.

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The difference in the apparent size of a supermoon versus a micromoon (the moon at apogee) is very obvious when you can see them side by side. (James Schaff)
The difference in the apparent size of a supermoon versus a micromoon (the moon at apogee) is very obvious when you can see them side by side. (James Schaff)

April's supermoon will be considerably closer than the previous full moon in March because the time of full moon and perigee coincide more closely. Full moon occurs at 10:33 p.m. Central Time on April 26 with perigee following about 12 hours later. When you gaze at the Pink Moon Monday night it will be nearly 18,500 miles (29,700 km) closer to the Earth than average and appear almost 8 percent larger. Unfortunately, it's difficult to recall the size of a typical full moon, so most of us won't notice the difference.

The year's closest supermoon happens on May 26th, with full moon and perigee separated by a little more than 9 hours. Supermoons are more than just a curiosity. Perigean full and new moons produce stronger tides than usual. Not only is the moon closer to the Earth, but the sun, Earth and moon are all in a row, with the sun and moon working together to pull on the planet.

The full moon appears flat and shadowless for the same reason gravel on a road blends together when the sun is at your back (right). In both cases, the sun shines straight down on the objects, hiding the shadows they cast. (Bob King)
The full moon appears flat and shadowless for the same reason gravel on a road blends together when the sun is at your back (right). In both cases, the sun shines straight down on the objects, hiding the shadows they cast. (Bob King)

When you observe the moon around the time of full through binoculars or a telescope, you'll be struck by its flat, pasty appearance. That's because the sun illuminates the moon exactly face-on like shining a flashlight directly in someone's face. Direct, shadowless lighting is often used on celebrity portraits to hide their wrinkles. On the full moon, it hides the rough outlines of craters, making the spherical orb look flat and two-dimensional.

When I turned 90° to the sun, so it shone off to the left, the shadows cast by the rocks made it easy to see their individual shapes. We see the same 90° angle lighting around half-moon. Notice how the shadows make the craters really pop. (Bob King)
When I turned 90° to the sun, so it shone off to the left, the shadows cast by the rocks made it easy to see their individual shapes. We see the same 90° angle lighting around half-moon. Notice how the shadows make the craters really pop. (Bob King)

I found a perfect demonstration of this lighting while walking on a recently-graded dirt road in the neighborhood. With the sun at my back, the exposed gravel in front of me blended into a nearly continuous, texture-less glow. With their shadows hidden by direct light, it was difficult to distinguish one stone from another. However, when I turned 90° to the sun, the shadows the rocks cast clearly defined their outlines, the same way the moon's craters show boldly when the moon makes a similar angle to the sun when we see a half-moon.

In this third example, I turned more toward the sun to simulate the moon's crescent phase. (Bob King)
In this third example, I turned more toward the sun to simulate the moon's crescent phase. (Bob King)

I continued this little experiment by pivoting to almost face the sun. Now, most of the gravel was in shadow with sunlight illuminating only the stones' tops and edges, the same way sunlight kisses the moon's edge when it's a crescent. I then made a complete circle, imitating the moon's orbit, and watched as the gravel went through "phases" of dark and light like a million mini-moons. Much fun!

If you find yourself on a gravel road with the sun low in the sky, do a slow rotation and you'll get a visceral feel for how lunar phases and moonscape lighting work.

And don't forget to catch the moonrise Monday night. To find out when it happens for your town, use this moonrise calculator.

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