Jupiter’s Back And It’s Good At Math

We have a new evening planet! Jupiter now rises in the faint constellation Libra the Scales around 11 p.m. You can use the planet to sketch out a giant triangle in the sky with the bright stars Arcturus and Spica. And I do mean a BIG: four fists fit between Jupiter and Arcturus, three between Arcturus and Spica and just under three between Spica and Jupiter. Stellarium

I stayed up way past my bedtime the other night. I knew it was late because Jupiter rose off to the southeast. What a pleasant surprise it was to see the big, bright planet up before midnight. Then I looked through the eyepiece of my telescope and got a second surprise: Ganymede, the largest of Jupiter’s moons, cast a neat, black shadow over the cloud tops of Jupiter’s north polar region.

Jupiter’s been edging its way toward evening ever since it reappeared in the morning sky after its conjunction with the sun last October. It takes months of Earth racing around the sun at 18½ miles a second to finally face in Jupiter’s direction before midnight. At conjunction, a planet lines up with the sun in the sky and is lost in the glare of bright dawn. But over time, Earth, the speedier planet, changes the viewing geometry, causing Jupiter to slowly separate from the sun. Last December, Jove stood low in a dark sky at dawn and by February high in the south. Now, it’s up after the 10 o’clock news.

Triangles can be classified according to the lengths of their sides.

Any bright star, planet or the moon can serve to direct our gaze to fainter but just as interesting objects in the night sky. Jupiter forms a very large triangle right now — call it the Jupiter Triangle if you like — with the bright stars Arcturus, located well above it, and Spica.

If you’re outside looking around 11:30 p.m. this week (10:30 p.m. by April 21), you can’t miss this thing of Euclidean beauty. While the sides are unequal now, making the figure a scalene triangle, just wait a while. Jupiter’s moving westward in Libra in the direction of Spica. By mid-June, two of the triangle’s sides will be equal, transforming it into an isosceles triangle.  (Admit it. No matter your age, math keeps coming back to haunt you).

Corvus the Crow (left) with Spica at upper right and Libra the Scales. Sidney Hall, Urania’s Mirror

You can use Jupiter and the triangle to help you find two small, fainter constellations: Libra the Scales and Corvus the Crow. This month, Jupiter sits practically in the middle of a small diamond of 4th magnitude stars representing Libra the Scales. Wait till it rises a bit higher to see if you can make out Libra’s “diamond” shape, then bop over to Spica and slide a fist and a half to the right (west) and you’ll come face to face with a crow. Corvus the Crow.

Neither pattern may look much like its namesake, but no matter. Once you find them, you’ll know two additional constellations in the sky. From these, you can then move on to others until you’ve tracked down the whole set of 88.  Like learning where the states are in the U.S. or the layout of countries in Africa or Europe, knowledge is a handy thing. Knowing even the simplest things builds a foundation that leads to a greater understanding of the universe. Your universe.

4 Responses

    1. astrobob


      A great word. That’s the other way to describe triangles — by their angles. Acute, obtuse and right.

  1. Aqua4U

    Last Sunday nite we had the first clear spot in a moonth, so astro pal Micah and I headed up the mountain for some viewing w/my home made 12.5″ f3.6 Newt. & 70mm Celestron binos. Leo at the zenith was irresistible.. so many galaxies there and below in Virgo! We spent hours clicking off faint galaxies – over 100! Micah is all over finding DIM stuff down to 14th mag. whereas I like the brighter ones I am most familiar with. Saw dust lanes in the Southern Pinwheel, M51 and star clouds in M101. Then Jupiter rose and we watched as Io went around the limb. We stayed long enough to see it come back into view on the other side of Jupiter, a LATE night session!

    Welcome back Jupiter! It’s good to see you again! aka – nice night light!

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