What If the Planets Were as Close as the Moon?

The Moon is about 384,400 km away and looks about half a degree wide. What would we see if each planet sat at that same distance instead? The visualization above shows an evening sky with the Moon swapped for each planet. The view uses a real field of view, so each one appears exactly as big as it would to your eye. The small dashed circle shows the real Moon’s size for comparison.

How big would each planet look?

For a sense of these angles, hold your hand out at arm’s length. Your little finger covers about 1°, and your closed fist about 10°. The Moon is half a finger. Jupiter would be about two fists.

Jupiter in place of the Moon

Jupiter would be about 21° across, 40 times wider than the Moon, covering as much sky as about 1,600 full Moons. Its cloud bands, the Great Red Spot and the white storm ovals would all be clear to the naked eye. It would also be brilliant. Jupiter’s clouds reflect about four times as much light as the Moon’s dark rock, and it would cover so much more sky that a “full Jupiter” would light the night roughly 7,000 times more than a full Moon.

Saturn in place of the Moon

Saturn itself would be about 17° across, but its rings would steal the show. The main rings span about 273,000 km, so at the Moon’s distance they would stretch about 39° across the sky, more than twice the planet’s width. Over the year we would see them open and close as the angle between us changed.

The ice giants and the small planets

Uranus and Neptune would be blue-green discs about 7.5° across, around 14 times wider than the Moon. Venus, nearly Earth’s size, would be 3.5 times wider than the Moon and dazzling white from its unbroken clouds. Mars would be twice the Moon’s width and clearly red. Mercury is only slightly larger than the Moon, and with its similar grey, cratered face it might take a while to notice the swap.

Could this really happen?

Not without dramatic consequences. At the same distance, a body’s tidal pull is proportional to its mass, and the planets are much heavier than the Moon:

• Mars would raise tides about 9 times stronger, and Venus about 66 times.
• Neptune and Uranus would pull over 1,000 times harder.
• Jupiter would pull about 26,000 times harder. Earth’s oceans would surge across continents, and the flexing would heat the planet’s interior, the same process that makes Jupiter’s moon Io the most volcanic world in the Solar System.

With a giant planet that close, it would really be Earth orbiting the planet, not the other way round. Around Jupiter at this distance, Earth would complete an orbit in about a day and a half. Jupiter would race through its phases in our sky every 36 hours.

Does the Moon look bigger near the horizon?

No. The Moon often looks huge when it is rising, but that is the Moon illusion, a trick of perception. Its size in the sky barely changes, and it is actually slightly smaller near the horizon, because you are then about one Earth radius farther from it. The visualization keeps every body at a fixed elevation and size.

About the visualization

Each body sits at the Moon’s mean distance, 384,400 km, measured centre to centre, so its apparent size is 2 × arctan(radius ÷ distance). The camera’s field of view is fixed at 50° vertically, about what a phone camera captures, so sizes on screen are true to life relative to the frame. Saturn’s rings run from the inner C ring to the outer edge of the A ring. The planets are lit by a Sun that has just set behind you, and their night sides blend into the twilight sky, as the Moon’s does. For the other planets side by side at true scale, see the Solar System planets in a row.

Sources

• Radii and masses: NASA planetary fact sheets.
• Saturn’s rings: inner C ring 74,658 km, outer A ring edge 136,775 km (NASA Saturnian rings fact sheet).
• Brightness: geometric albedos from the NASA fact sheets (Jupiter 0.54, Moon 0.12).