Neptune reaches Opposition

This week, on August 27, the planet Neptune reaches opposition.

On August 27 Neptune reaches opposition in Aquarius, making it visible all night. Credit: Starry Night Software

When a planet is in opposition, it lies directly opposite the sun in Earth’s sky. It is highest in the sky when the sun is lowest, which is local midnight. When Daylight Saving Time is in effect, this is close to 1 a.m. local time.

Because Neptune is directly opposite the sun. it rises at the same time as the sun sets, and sets at the same time as the sun rises, so is visible all night.

Now that Pluto has been demoted to “dwarf planet” status, Neptune is the most recent planet to be discovered, on September 23, 1846, and the farthest planet from the sun, at an average of 2,798,310,157 miles (4,503,443,661 km.)

As planets go, Neptune is extremely dim, requiring at least a binocular to become visible. Even in a powerful telescope, it is a tiny blue-green disk with no detail to be seen. Unless you look carefully, you could easily mistake it for a star.

In fact, that happened several times before Neptune’s official discovery. Most famously, Galileo twice observed Neptune while studying Jupiter’s moons in 1612 and 1613, but mistook it for a star both times.

After the discovery of Uranus by William Herschel in 1781, mathematicians calculated the possible location of another planet farther from the sun, but no one looked seriously for it until 1846. The first to actually spot it was German astronomer Johann Gottfried Galle on September 23.

Because it takes Neptune 164 years to circle the sun, it spends an awfully long time in any one constellation. For example, Neptune has been in Aquarius since January 24 2011 and won’t move on into Pisces until May 22 2022. In fact, it has only just completed its first trip around the sun since its discovery, and is again very close to the spot where it was discovered.

Since Neptune is so far from the Earth, it presents too small a disk to be studied well with even the largest telescopes. The only good view we’ve ever gotten of Neptune was in 1989 when the Voyager 2 probe passed within 2740 miles (4400 km.) of Neptune’s cloud tops. At that time it recorded two large blue spots in Neptune’s atmosphere, apparently similar to the Great Red Spot on Jupiter. Voyager 2 also confirmed the existence of a faint ring around Neptune, a ghostly echo of Saturn’s ring system.