Jupiter And The Moon So Close, Yet So Far Apart

Watch for the moon and Jupiter in conjunction overnight and see if you can imagine the space between them. Stellarium

If you’re up really early tomorrow (about 1 a.m.), you’ll see the waning gibbous moon paired up with Jupiter in the southeastern sky. The two will about 3° apart, so eye-catching to be sure. You’ll also see them a few hours later at dawn, when they’ll float higher in the southern sky.

Conjunctions are both a joy to look at and one of the best illusions in the night sky. During a conjunction we see two celestial objects along nearly the same line of sight, so they appear close together. Since all objects, even the moon, are far from the Earth, we can’t sense their true distances. On Earth, sensing distances is far easier because cues in the landscape as well as our double-eyed stereo vision help our brains figure it out.

But when it comes to celestial objects, we can’t see them in three dimensions; our stereo vision simply doesn’t work at those distances. We also lack earthly cues.

Tomorrow morning, the moon and Jupiter may appear to be close together, but we know from what we’ve learned about the solar system that the moon is much closer. They only appear close together because we see them along the same line of sight, the same way you might look past a nearby tree to see a ship in the distance. Their true separation is 460.3 million miles (740.8 million km).

This diagram shows tomorrow morning’s conjunction of the moon seen from above the plane of the solar system. The Earth, moon, Jupiter and the moon’s orbit are approximately to scale. Jupiter however is shown much too close. It’s actually about 60 times farther away — way, way off to the right.  Bob King

I thought it would be fun to try and show their true distances in a diagram but realized from the start I’d need a VERY long sheet of paper for that. So I shrunk the Earth, moon, the moon’s orbit and Jupiter down and made a few measurements. I have a 24-inch monitor. When the illustration is enlarged so the distance between the moon and Jupiter equals 12-inches, the distance between them to scale comes to 7,680,000 miles. MUCH too close, but then I knew that.

To show the true distance between them at the scale I chose, you would have to lay 60 (sixty) copies of the above diagram end-to-end with the moon at one end and Jupiter at the other. That’s 60 x 12-inches or 60 feet, the distance from my chair down the stairs and out the front door. Not surprisingly, at that scale, the Earth, moon and Jupiter would be nothing more than pinpricks.

The point is, there’s a lot of space out there requiring power of the imagination to perceive. Give it a try at tomorrow morning’s conjunction.