How far is each planet from Earth?
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. For example, the planet Venus is about 42 million kilometers (26 million miles) from Earth at its closest point and about 258 million kilometers (160 million miles) away at its most distant point. This diagram, with Earth as the blue dot, shows Venus at these two places in its orbit.
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, right, 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 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 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.
|Planet||Distance from Sun in AU|
To find the 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 - 0.72 = 0.28 AU. To get the number in kilometers, multiply by the conversion above: 0.28 x 150 = 42 million kilometers. Another example: Jupiter is 5.2 AU from the Sun and Earth is 1 AU from the Sun. So the distance between Jupiter's orbit and Earth's orbit is 5.2 - 1 = 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 elliptical-shaped 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. But the numbers above are generally what's used in introductory astronomy classes.
If you'd like to see which planets are closer to Earth right now, you can look at the Solar System Live website, 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.
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