We often recite the names of the planets with confidence as if they were neatly lined in a row, one placed directly next to its neighbor. Outward from the fiery sun first comes little Mercury, then brilliant Venus, then Earth and red Mars. Just beyond them is the largest planet, Jupiter followed by Saturn, the one with rings. On its other side are the two smaller gas giants, the somewhat mysterious duo of Uranus and Neptune. All neatly ordered. All accounted for. But that really doesn’t convey the true spacing of the planets in the vastness of the solar system, especially in its outer regions. They are not in a row and they are not next to each other.
From our vantage point on Earth
We can see upward of five bright planets. Tonight, it will be only four.
Venus shines conspicuously low in the west shortly after sunset. It won’t rise much higher before it drops quickly towards the set sun as late December approaches.
Mercury hides in the bright morning twilight until it climbs ahead of the sun in late October. Look to the southeast after Oct. 20 for the little world as it brightens above the horizon 40 minutes before sunrise.
Mars, Jupiter, and Saturn, when they are not close to the sun in the sky, are relatively easy to see. However, Mars, because it has now moved behind the sun, won’t be seen again until the final few mornings of 2021 — and only then with difficulty.
Look in the south 90 minutes after sunset for the brightest point of light, Jupiter. Fully extend your arm and make a fist. Place Jupiter on its left side. Saturn, shining not nearly as brightly, lies on its right.
Uranus and Neptune are next in the planetary line up after the easy to see Saturn. Why can’t they be found?
Because they are smaller than Saturn, Uranus and Neptune both show only about 10% the surface area for reflecting sunlight as Saturn. Hence, they are going to be substantially dimmer. Add to that their greater distances, and it is plain to see why they are not bright, unlike Saturn.
Uranus can be seen by the unaided only under the best of sky conditions. Neptune, lying roughly twice as far, is always invisible in our sky.
From Neptune’s vantage point
The appearance of the night sky from that farthest planet (some 30 times the distance from Earth that the sun lies from Earth) is similar to the celestial sky as seen from Earth only in that the stars appear exactly the same. However, there are no bright planets in the Neptunian sky. its nearest neighbor tonight is Uranus which is always too dim to be discerned by the unaided eye.
The only two planets seen are Jupiter and Saturn and they are found with difficulty as faint points lying, at most, “two fist widths” from the blazing sun. (Even at Neptune’s great distant, the sun is still far too bright to view directly and must be shielded.)
Neptune may be the most isolated planet, but it is far from being lonely. It carries an entourage of moons as it makes its 165 year endless journey around the sun.
Triton, by far its largest moon, appears about the same size as our Moon does in our sky. (Its actual diameter is less than three-fourths of our Moon’s.) It is nowhere as bright, though, glowing as a dim, small globe in the blackness of space.
At least seven other moons, all much smaller the Triton, accompany the planet. They each vary in brightness as they orbit Neptune, occasionally shining as brightly as Jupiter does in our sky, but most times appearing as faint points, wavering on the edge of visibility.
Now, revisit the planetary line up. Are they lined in a row? Is Neptune next to Uranus?
John Goss is the past president of the Astronomical League.