Jupiter and Saturn made their closest approach to Earth in August. The two gas giants still rule the night sky, with Jupiter being the brightest speck of light throughout the night. However, over the coming months, they will fade as their orbit takes them closer to their conjunction (or the far side of the solar system). As Jupiter and Saturn recede, it’s time for Neptune to make its closest approach to Earth. Neptune will be at its opposition on September 14. Here is everything you need to know about this astronomical event.

1. What is an opposition?

In celestial mechanics, a planetary opposition occurs when the Earth lies between the Sun and the planet, with Earth being on the same side of the planet, as shown below:

Neptune is Going to Make Its Closest Approach to Earth This Week: 3 Things to Know. 1
The concept of opposition and conjunction in astronomy

Thus on September 14, Sun, Earth, and Neptune will almost align, with Earth in the middle. This optimal positioning occurs when Neptune is almost directly opposite the Sun in the sky. Since the Sun reaches its greatest distance below the horizon at midnight, the point opposite to it is the highest in the sky at the same time.

At around the same time that Neptune passes opposition, it also makes its closest approach to the Earth – termed its perigee – making it appear at its brightest and largest. Neptune’s orbit lies at around 29 AU from the Earth (1 AU = 150 million km), and hence the angular size won’t vary much over the course of conjunction, and opposition, compared to Jupiter that lies at 4.2 AU.

Neptune at opposition 2021

2. How close will be Neptune?

On September 14, Neptune will be at a distance of about 4.32 billion km (28.92 AU). That’s about 11.9 million km closer than the average distance from Earth. The angular size of Neptune will be 2.4 arcsecs.

3. How to see Neptune?

Neptune is Going to Make Its Closest Approach to Earth This Week: 3 Things to Know. 2
Neptune’s position on September 14 (Image: Dominic Ford, in-the-sky.org)

Since Neptune will lie opposite the Sun, it will rise at dusk in the constellation of Aquarius. However, even at its closest approach to the Earth, it is impossible to see it with naked eyes as the apparent magnitude of Neptune will be +7.8, and unaided human eyes can see up to mag +6.0. You’ll need a telescope to see the planet.

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