They've been our constant companions since last summer, but Jupiter and Saturn will soon disappear from the evening sky. Already, Saturn is virtually impossible to see, hidden deep in the twilight glow. Jupiter struggles on into early next week, then it's curtains for the big guy. Saturn will be in conjunction with the sun on Jan. 24 followed by Jupiter on the 29th.

Outer planets such as Mars, Jupiter and Saturn shift from the evening sky into the morning sky when the sun passes them. Remember that the apparent motion of the sun to the east (left in the northern hemisphere) is caused by Earth's revolution around it. Right now, Jupiter lies east of the sun. After the sun passes it on Jan. 29, the planet will appear to its west and rise before the sun at dawn. (Stellarium with additions by the author)
Outer planets such as Mars, Jupiter and Saturn shift from the evening sky into the morning sky when the sun passes them. Remember that the apparent motion of the sun to the east (left in the northern hemisphere) is caused by Earth's revolution around it. Right now, Jupiter lies east of the sun. After the sun passes it on Jan. 29, the planet will appear to its west and rise before the sun at dawn. (Stellarium with additions by the author)

The same way Jupiter and Saturn passed closest to each during their own conjunction last December, each planet in turn will pass closest to the sun on the dates above. Obviously, the sun's brilliance means they'll be swamped in its glow and invisible for a time until they reemerge in the dawn sky. That won't happen until the end of February. When you see them again you'll be amazed by how much Jupiter has outpaced Saturn. On Feb. 28 they'll be 8 degrees, or nearly a fist, apart!

From the moon, where the stars shine day and night, you might see Jupiter right up to conjunction day ! (Stellarium)
From the moon, where the stars shine day and night, you might see Jupiter right up to conjunction day ! (Stellarium)

From the moon, stars are visible day and night because there's virtually no atmosphere to scatter the sun's light and turn the sky blue. I suspect that even on Jupiter's conjunction day if you covered the sun with you hand and carefully looked 1/2 degree just to its south you'd see the planet gleaming. It's fun to use our imaginations this way, but you also have another option — the orbiting Solar and Heliospheric Observatory (SOHO).

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Jupiter was near conjunction in Sept. 2016 when SOHO's coronagraph took this photo. We can easily see the planet, the sun's corona and many stars. (NASA/ESA)
Jupiter was near conjunction in Sept. 2016 when SOHO's coronagraph took this photo. We can easily see the planet, the sun's corona and many stars. (NASA/ESA)

Multiple times a day SOHO takes photos of the sun and its nearby environment with a coronagraph, a specialized instrument that uses a mask to block the solar glare so astronomers can see all the cool things happening right up to its edge. These include enormous, flame-thrower-like particle bursts called coronal mass ejections (CMEs), daytime comets as well as planets when they're at or near conjunction with the sun. When Saturn and Jupiter are at conjunction, go to the SOHO site and click on the LASCO C3 image, and you'll see each planet as a bright "star" next to the hidden sun. Or you can begin earlier. Saturn made its first appearance in the LASCO C3 on Jan. 15 and Jupiter arrives on the 19th.

A bright new face will briefly replace Jupiter and Saturn at dusk from now until month's end — Mercury. Midway between Jupiter and Saturn in brightness, Mercury will hover low in the southwestern sky. For the next few days you can spot it between 45 minutes and an hour after sunset about 4 degrees (two fingers held together horizontally at arm's length) above the horizon. Its elevation increases in the coming week to 10 days to 8 degrees. This will provide multiple opportunities to see the quick-footed planet in case you're battling clouds.

Although Mercury is nearly twice as close to the sun as Venus it's not nearly as hot. With a surface temperature of 330 degrees (167 C) you could boil tea in a second, but Venus is far more torrid. There the temperature sizzles at 900 degrees (465 C), hot enough to melt zinc. The planet's extremely dense atmosphere is composed of 96.5 percent carbon dioxide. CO2 traps heat from the sun, driving a non-stop greenhouse effect that made the temperature skyrocket.

Mercury is an inner planet. As it swings from one side of the sun to the other during its 88-day orbit we see different amounts of the planet illuminated. Right now, Mercury looks like a waning  gibbous moon when magnified in a small telescope. (Bob King)
Mercury is an inner planet. As it swings from one side of the sun to the other during its 88-day orbit we see different amounts of the planet illuminated. Right now, Mercury looks like a waning gibbous moon when magnified in a small telescope. (Bob King)

Mercury is the smallest planet with a diameter of 3,032 miles (4,880 km), about twice as big as the dwarf planet Pluto. Because it takes just 88 days to circle the sun, Mercury never sticks around for long. We see it for a few weeks at dusk before it heads back into the solar glare. A couple weeks later it's back at dawn. And the fact that it's close to the sun means it never strays far from its glow. For observers in mid-northern and mid-southern latitudes Mercury is only visible during twilight. During the course of a year the planet goes around the sun more than four times, bouncing from evening to morning sky and back again like a ping pong ball.

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