Neptune Will Make Its Closest Approach To Earth Tonight: 3 Things To Know

It is September and it's the time for the Roman God of Sea to make a close visit to Earth. Here are 3 important things to know about Neptune's opposition tonight.

1. What Is Opposition?

In astronomy, two celestial objects are said to be in opposition when they lie on opposite sides of the celestial sphere as observed from a given body. Now since Neptune is at opposition, this means that Sun, Earth and Neptune will lie in the same line with Earth in between the two (as illustrated in the diagram below). At its opposition, a planet almost reaches its perigee (closest point to Earth) and appears biggest and brightest.

The illustration showing different terms of positional astronomy including opposition. (Image: Wikipedia)


2. How Close Will Be Neptune?

The average distance of Neptune from Earth is about 30.27 AU (1 AU = 150 million Km, the mean distance between Earth and Sun). At the opposition, Neptune will be at 28.93 AU. This means the gas giant will be 201 million Km closer to Earth than its average distance.

3. How To Watch This Event?

When a planet is at opposition, it is visible throughout the night. This happens because when Neptune lies opposite the Sun in the sky, the solar system is lined up so that Neptune, the Earth and the Sun form a straight line with the Earth in the middle, on the same side of the Sun as Neptune.

The planet will make its closest approach exactly at 23:43 IST (18:13 GMT) on September 7. Neptune will be well placed for observation, in the constellation Aquarius. It will be visible for much of the night, reaching its highest point in the sky at around midnight local time.

Image: Dominic Ford (

Neptune will have an apparent magnitude of +7.8 so it won't be visible with the unaided eye. You'll need a telescope for that.

Also Read: Top Astronomy Events For September 2018

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