Astro Bob: Catch Mercury at dawn
The smallest, swiftest planet puts on a fine show the next couple weeks.
For the next two weeks Mercury will fluff its feathers at dawn, making its best morning appearance since February for northern hemisphere observers. Of course, the planet has no feathers. But it moves so swiftly around the sun, we might envision it as the peregrine falcon of the solar system
Mercury rises nearly due east and stands between 5 degrees and 8 degrees high an hour or so before sunrise. One of the many things I love about October are the later sunrises — after 7 a.m. from most locations. That makes it easier to squeeze in a little dawn skywatching without taking a serious hit to your sleep.
Given the planet’s low altitude, find a place with an unobstructed view to the east to ensure a successful hunt. Binoculars can also help, especially if there’s haze about. You can use the map or check exactly where to look on the interactive Stellarium web page at stellarium-web.org . Click the time readout on the lower right side of the page and change the time to an hour before sunrise. Then zoom and drag the star map with your mouse to face east. I plan to head down to Lake Superior for a look. If I’m lucky and a windless morning turns the lake to glass, two Mercuries will glimmer back.
And as long as you’re going to the trouble to see the planet, consider sticking around for the sunrise. Simple to see and yet among the most satisfying of astronomical sights. I will never live long enough to have my fill.
Mercury whips around the sun every 88 days. We last spotted the speedster in late August in the west after sunset. Six weeks later, it’s swung around to the other side of our star and shines in the morning sky. Mercury’s appearances are brief because it orbits quickly and spends so much time in nearly the same line of sight as the sun, so it’s difficult to impossible to pick out in the glare. That’s why it’s worth seeking when it’s at or near greatest elongation, when it stands as far east or west of the sun as possible.
The planet starts out the week at a respectable magnitude 0.5, about as bright as red-hued Betelgeuse in Orion. But it quickly brightens. On Thursday, Oct. 6, Mercury will shine at −0.3, brighter than Arcturus. Before it disappears in the solar glow around Oct. 20 it will even rival Sirius, the brightest star of the night. That doesn’t mean it will LOOK as bright as those luminaries. We have to take into account its low altitude and the glow of twilight, both of which dilute its light.
The planet’s changing phase is the reason for its steady increase in brilliance. When Mercury first appears in the morning sky, it’s situated almost directly between the Earth and sun and in crescent phase. Then it swings to the east (to the right in the diagram) and fattens to half. Just like our own moon, Mercury gets brighter as its phase increases towards full, the reason it keeps getting brighter in the coming weeks.
Unlike the moon, its apparent size changes dramatically over the course of its orbit. Mercury appears largest when it’s a crescent because it’s closest to the Earth, then shrinks to a teensy dot when it’s on the opposite side of the sun and most distant from us.
Mercury orbits quickly because it’s so close to the sun. It’s also the smallest planet, with a diameter of just 3,031 miles, or 1.4 times the size of the moon, and home to some of the most extreme temperatures in the solar system. Daytime highs on its surface climb to around 800 degrees Fahrenheit and nighttime lows dip to −290 degrees. Mercury lacks an atmosphere, so all the heat it sucks up during the day rapidly dissipates at night.
Although the planet makes one spin on its axis every 59 days, a day there lasts much longer. Mercury’s rapid revolution slows down the daily motion of the sun across the sky, increasing the length of the day from a nominal 29.5 Earth days (half of 59) to 176! No wonder it gets so hot and so cold.
When you set your alarm to meet Mercury face to face, try to imagine what a hellish and barren place it is compared to Earth, where you can still grab a cup of coffee almost anywhere anytime.