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# Distance in the Solar System

The scale of the planets is tiny compared to the scale of the Solar System. The distance from Earth to the moon is 384 thousand kilometers, or 9.6 times Earth's equatorial circumference. The Sun is 150 million kilometers away, or 390 times the distance of the Moon from Earth, and 3,743 times Earth's circumference. When we speak of the distances between the planets, we are speaking of a scale that dwarfs not only the scale of the planets, but also the scale of a planet's system of moons.

The basic unit of distance for the Solar System is the Astronomical Unit (AU). Roughly speaking, this is the distance of Earth from the center of the Sun. More precisely, the AU is the length of the semimajor axis of the Earth-Moon system's orbit around the Sun. The AU is approximately 1.50×108 km (a more precise value, accurate to 50 meters, is given in the Basic Values table). This is 100 billion times our human scale, or the ratio of the size of a house to the size of an atom.

In terms of Earth's orbit, Earth's equatorial radius is 4.0×10-5 AU, Jupiter's equatorial radius is 4.8×10-4 AU, and the Sun's equatorial radius is 4.6×10-3 AU. The size of each planet's system of moons is also much smaller than an AU. The Moon is on our doorstep at 2.56×10-3 AU, so the Earth-Moon system could fit inside the Sun, comfortably if it weren't so hot. The outermost of the four Galilean moons of Jupiter has an orbit with a semimajor axis of 0.013 AU. Saturn's moon Titan is in an orbit with a semimajor axis of 8.1×10-3 AU, and the outer edge of Saturn's A ring, the outermost of the rings that is easily seen from Earth, is 9.1×10-4 AU in radius, which is little more than one-third of the Moon's distance from Earth.

The terrestrial planets have distances from the Sun in the AU range: the semimajor axis of Mercury is 0.39 AU, that of Venus is 0.72 AU, and that of Mars is 1.52 AU. Once we turn to the giant planets, we jump to a length scale of tens of AU. Jupiter's semimajor axis is 5.20 AU, Saturn's is 9.54 AU, Uranus's is 19.19 AU, and Neptune's is 30.07. The Kuiper belt of planetoids ranges from 30 to 50 AU. Finally, the Oort cloud, which gives birth to the long-period comets, is more than 100 AU from the Sun.

The illusion that light moves instantaneously is lifted for light traveling through the solar system. Light travels 1 AU in 499 seconds (8 minutes and 19 seconds). As a consequence, large time delays occur in communicating with interplanetary spacecraft. The time delay for communicating one-way with the Mars Rover ranges between 4 and 21 minutes, and this delay for communicating with the Cassini spacecraft at Saturn ranges between 71 and 88 minutes.

We can traverse interplanetary distances by spacecraft, but we are unable to achieve velocities that are dramatically larger than the orbital velocities of the planets. As a consequence, travel to any planet takes months or years. Travel to Mars takes 6 months. It took 7 years for the Cassini spacecraft to travel to Saturn in a trip that required gravitational boosts through close passages of Earth, Venus, and Jupiter.