Neptune in Opposition – Any One Observing?


Neptune will be in opposition on 3rd September 2016, however the occurrence is so uninteresting that hardly any amateur astronomer is known to pay attention to them. Being the outermost planet in our solar system makes it exception to the general rule which says that the best time to observe outer planets is during their opposition.

During the opposition, Neptune will be positioned directly opposite to the Sun as seen from the Earth, as a result, the planet is as close to the Earth it will ever come. This occurrence repeats every 367.5 days on average, that is 2.5 days later each successive year. This makes Neptune rise near the time of Sunset in the sky and set near the Sunrise, making it visible all night long. Neptune will be highest in the night sky at the time of midnight.

Why exactly Neptune Oppositions are Uninteresting

Neptune orbits the Sun at the distance of 30.1 Astronomical Units (AU), making it almost 30 time further from the Sun than Earth, which means that at the time of an opposition, Neptune is only about 6% closer to Earth as compared to the time when it is farthest (at Superior conjunction). This results in only a small amount of difference in the brightness of Neptune between the two events. In fact, during an opposition of Neptune on September 3rd 2016, it will be shining at 7.8 magnitude while at the last superior conjunction on February 29, 2016, it was only a tiny bit fainter at 7.97 magnitude. It is this very little change in the brightness from the time when Neptune shines at its brightest and faintest that makes observing Neptune at an opposition a hardly a worthwhile effort.

If you are still interested in observing Neptune during this opposition, read on.

Finding and Observing Neptune

Neptune currently can be found in the constellation of Aquarius very close to the south-west of the star Lambda Aquarii (mag. +3.7). Finding and observing neptune is a challenging but rewarding exercise. Being farthest farthest planet in the solar system makes it the faintest planet and the only planet which requires a telescope to observe at all times. Observers with some experience and familiarity with using telescopes and star charts can attempt locating it in the night sky.

Tools You will Need

A telescope or a binoculars (Neptune is too faint to be seen with naked eyes).
A planisphere or your smartphone with any good star gazing app installed. You will need this to locate the constellation in which Neptune resides.
A detailed finder chart of Neptune for 2016 such as the one here. This maps show stars as faint as 9th magnitude.
A dim red colored torch which you will be needing to see star charts or planisphere in dark. The dim red colored light will allow you to see the maps while still allowing you to maintain dark adaptation of your eyes.

Steps To Follow

Use your planisphere or a star gazing app installed on your smart phone to locate aquarius in the night sky. It will be towards south.
Locate the 3rd magnitude star Lambda (λ) Aquarii and point your telescope or binocular towards it.
Use the finder chart to star hop and locate neptune. It will have a dull greenish blue appearance and will be 7.8 magnitude in brightness. In small to medium sized telescope, the planet will not be big enough to see the disk. However, the distinct color will help you identify it.
A Treat for the Arm Chair Observers
If you are really not interested in making the effort to go out and observe Neptune, you can try ‘armchair astronomy’ by logging watching the opposition of Neptune being webcast live by Slooh telescope on 4th September, 2016 at 4:30 AM Indian Standard Time (IST).

Vikrant Narang
Sr. Educator
Astro Tourism India Pvt. Ltd.
(Note: Astro Tourism India Pvt. Ltd. is a Sister Concern of SPACE)

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