## How far is each planet from Earth? (Intermediate)

We get lots of emails asking this, but it's actually a difficult question to answer because the planets are moving in their orbits all the time, and so the distance from each planet to Earth is constantly changing. When both planets are on the same side of the Sun, and form a line with the Sun, then they are closest to each other. When two planets are on opposite sides of the Sun, and form a line with the Sun, then they are farthest away.

An inferior planet (a planet whose orbit is interior to Earth's orbit) is farthest from Earth near the time of superior conjunction and closest to Earth near the time of inferior conjunction. The diagram below (from this page), with Earth as the blue-green dot, shows a gray circle representing an inferior planet (Mercury or Venus) at these places in its orbit. For superior planets (like Mars), shown as the red circle in the diagram above, the planet is closest to Earth at about the time of opposition, and it is farthest from Earth at about the time of conjunction. However, since the planets' orbits are ellipses (not perfect circles), and they are not in exactly the same plane, the planet's minimum or maximum distance from Earth may not occur at exactly the same time as opposition or conjunction.

Usually when people ask this question, what they mean is "What is the distance between the orbit of Earth and the orbit of each planet?" or "What is the closest that each planet comes to Earth?" (These are essentially the same question, because the planets can't get any closer than their orbital spacing allows.) You can compute this in a rough way by assuming that the orbits are circular and coplanar, and looking at the planet-to-Sun distance for each planet. Since the distances are so large, we usually express them in Astronomical Units (AU). (An AU is the average distance from Earth to the Sun, about 150 million kilometers or 93 million miles.) The table below lists the distance of each planet from the Sun in AU. (Numbers were taken from this page.)

 Planet Average distance from Sun in AU Mercury 0.39 Venus 0.72 Earth 1.00 Mars 1.52 Jupiter 5.20 Saturn 9.58 Uranus 19.20 Neptune 30.05 Pluto (dwarf planet) 39.48

To find the approximate distance between the orbits of two planets in AU, subtract the two planet-Sun distances. For example, Earth orbits at 1 AU from the Sun and Venus orbits at 0.72 AU from the Sun. The difference between these two distances is 1.00 - 0.72 = 0.28 AU. To get the number in kilometers, multiply by the conversion above: 0.28 × (150 million kilometers) = 42 million kilometers. Another example: Jupiter is 5.2 AU from the Sun and Earth is 1.0 AU from the Sun. So the distance between Jupiter's orbit and Earth's orbit is about 5.2 - 1.0 = 4.2 AU or 630 million kilometers. You can use this process for any of the planets using the numbers above.

For completeness, I should point out that the circular orbit assumption is better for some planets than others. For example, Mars has a fairly eccentric elliptical orbit, which means that its "closest-approach" distance changes. That's why in August 2003, Mars was closer to Earth than it has been in about 60,000 years (It won't be that close again until the year 2287!). But the numbers above are generally what are used in introductory astronomy classes. And that's why the Earth-Venus distance calculated in the previous paragraph (42 million kilometers) is slightly greater than the actual minimum Earth-Venus distance (38 million kilometers) - the planets' eccentric orbits can allow them to get slightly closer than the distance that one finds from the simple calculation.

If you'd like to see which planets are closer to Earth right now, you can look at the Solar System Simulator or Solar System Live websites, where you can choose a time and then look down on the Solar System from above to see where the planets are.

If you're technically inclined and want exact numbers, you can use the JPL Horizons system to select a planet, select a time range, and select "Observer range and range-rate" as an output quantity. You will get a list of times along with the Earth-object distance in AU.

This page was last updated by Sean Marshall on March 31, 2016.

#### Lynn Carter

Lynn uses radar astronomy to study the planets, especially Venus. She got her PhD in Astronomy from Cornell in Summer 2004 and is now working at the Smithsonian in Washington D.C. on the Mars Express radar.

## Our Reddit AMAs

AMA = Ask Me (Us) Anything