Astro Bob: Meet Mercury, a strange little planet

Home to magnetic tornadoes and 800-degree temperatures, Mercury is visible at dusk through mid-month. The Full Pink Moon also joins the scene on April 5.

Mercury orbit and phases
Mercury shows phases like the moon because it orbits the sun (every 88 days) interior to Earth's orbit. When off to one side of the sun we see half the planet in sunlight. On the opposite side of the sun from Earth it's fully lit like the full moon. When it passes between us and the sun we see a thin crescent.
Contributed / ESO, NASA Earth photo with additions by Bob King

Whenever Mercury decides to show itself I like to be there. That usually happens a couple times a year at dusk and dawn. Mercury always appears in twilight and never a dark sky because it orbits close to the sun and can't escape its glow. Our view is limited to an hour-long window after sunset or before sunrise. In late winter and spring, Mercury stands at a steep angle to the sun after sunset, so it's a little higher up than normal and easier to spot. Similar circumstances occur at dawn in late summer and autumn.

Mercury Venus and Orion
Mercury joins Venus at dusk during the first half of April. The best time to see the elusive planet is from 40 minutes to an hour after sundown low in the northwestern sky.
Contributed / Stellarium

Most of us have seen Venus, Mars, Jupiter and Saturn. But Mercury's persistent low altitude makes it elusive despite it being brighter than Sirius at times. Thanks to favorable viewing circumstances you can beat the odds. For the next 10 days, until mid-April, the planet puts on a fine appearance at dusk for northern hemisphere skywatchers.

Solitary Mercury
Mercury will be the only bright "star" low in the northwest in April. It always hugs the twilight glow.
Contributed / Bob King

To spot it, you'll need a location with an unobstructed view of the northwestern sky. Start looking about 40 minutes after sunset when Venus comes into its own and can offer a helping hand. Mercury gleams two fists below and to the right of the planet.

I always bring binoculars when seeking planets or comets buried in twilight. Focus Venus to a sharp point, then lower the binoculars and slowly sweep back and forth around Mercury's position. Once you've found it, it's pretty easy to see the planet with just your eyes. Mercury will look like a solitary "star" a couple fingers above the horizon.

Mercury messenger color.jpg
Mercury revolves around the sun every 88 days and takes 59 days to complete one rotation. Slow rotation coupled with its rapid revolution makes for double sunrises from some parts of the planet. When Mercury is closest to the sun and moving fastest, the morning sun will rise, then reverse direction and set before rising for good a second time. The same scenario can occur at sunset. Because its axis is tipped just 2° craters near the planet's north pole are permanently shadowed and harbor ice.
Contributed / NASA, JHUAPL, Carnegie Institution

Over the next two weeks, Mercury will slowly fade as its phase narrows from gibbous to half to crescent. Unlike Venus, which is almost identical in size to the Earth, Mercury is only 1.4 times as big as our moon and smaller than the two largest moons in the solar system — Jupiter's Ganymede (3,274 miles) and Saturn's Titan (3,200 miles).


Buckled crust
After its formation Mercury cooled and shrunk, which caused its crust to buckle and form tall, sinuous cliffs called scarps. This one is named Beagle Scarp. Mercury's craters are named for composers, artists and writers.
Contributed / NASA, JHUAPL, Carnegie Institution

At a casual glance Mercury's heavily cratered surface looks like the moon's, but it has its own unique land forms including long cliffs called scarps. In the distant past the planet cooled and shrank. To accommodate its shrinking volume, gravity forced the crust to buckle, creating standalone cliffs up to 620 miles (1,000 km) long and 1.8 miles (3 km) high. Mercury also shows abundant evidence of past volcanic activity with vast lava plains and "ghost craters" buried under volcanic deposits.

No surprise, it's also really hot there. Almost three times closer to the sun than the Earth, Mercury's the surface heats up to 800° F (427° C) during the day and dips to -290° F (-179 °C) at night. With a daily variation of nearly 1,100° it has the most extreme surface temperature range of any planet.

Why? First, it's much closer to the sun, which is seven times brighter there and three times the apparent size we see it from Earth. Second, the planet has virtually no atmosphere. With no air for insulation, heat radiates back into space during the planet's lengthy night. Mercury's slow rotation (59 days) coupled with its rapid orbit (88 days) conspire to make a full day-night cycle 176 Earth days long!

Mercury core
Mercury's metallic core is huge compared to the other rocky planets. Molten iron flows in the outer core produce a magnetic field around the planet. Presumably, a compass would work there similar to how it points on Earth, which has a much stronger magnetic field.
Contributed / NASA's Goddard Space Flight Center

If that's not strange enough, the planet has an iron core that takes up nearly three-quarters of its interior. Flows of molten iron in the outer core create a weak magnetic field (1% that of Earth's) much as Earth's molten outer core generates our own planet's magnetic envelope. Here that field can couple with the solar wind and produce the aurora.

Although Mercury's thin atmosphere precludes aurora formation, its magnetic field interacts with magnetic fields in the solar wind to spawn magnetic tornadoes that funnel particles in the wind down to the surface of the planet. Some of these invisible tornadoes are up to 500 miles (800 km) wide. They scour atoms from the bare rock and whoosh them to high altitudes where they replenish Mercury's scant atmosphere with fresh oxygen, sodium and hydrogen.

You can picture all this in your mind's eye as you seek the planet at dusk in the coming nights. If you have a telescope, you'll also be able to watch its phase changes, too.

Find the Pink Moon

Lift bridge moonrise
A nearly full moon rises within the framework of Duluth's Aerial Lift Bridge in March 2015.
Contributed / Bob King

April is the month of the Pink Moon, named for moss pink, a type of phlox flower that grows in the northeastern U.S. Here in northern Minnesota the native Ojibwe people know it better as the Sugar-bushing Moon . This is the time of year when maple sap is collected and boiled down to make maple syrup and candy.

The full moon will rise nearly due east in Virgo around sunset (find your sunset time at ) on Wednesday, April 5, and shine all night. Find a place with a good view in that direction and arrive at least 10 minutes to get comfortable. Then lean back and receive this simple gift.


"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune. You can reach him at
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