Neptune is now best positioned for 2021 as it hits opposition on September 14, when the Sun, Earth and Neptune align. Neptune sits opposite the Sun in the sky, hence the term “opposition”. This is your best opportunity to track down and observe the eighth and currently most distant known planet in the solar system.
In contrast, Neptune is an unimaginable distance of 4.3 billion kilometers (2.67 billion miles), nearly 29 times the average distance between Earth and the Sun (a distance called an astronomical unit). From these dark and icy depths of the solar system, the sunlight reflected by Neptune takes four hours to strike our retinas! Neptune is described as an “ice giant” along with Uranus, the seventh planet from the Sun. Neptune is a large body, with a radius of 24,622 kilometers (15,299 miles), making it four times the width of Earth.
Despite its impressive physical presence, Neptune’s great distance from us diminishes our terrestrial vision so much that Neptune is the only planet that is not visible to the naked eye. Additionally, a small to medium aperture telescope may have difficulty showing its relatively small disk. However, an observer half full of glass would put a positive spin on things and point out that a humble pair of binoculars is often all that is needed to locate magnitude +7.8 Neptune and an observatory class instrument does not. is not necessarily necessary to observe or image the ice giant from a distance.
Neptune is currently found among the stars in northeastern Aquarius, near the border with Pisces to the north. It can be located about 4.3 degrees northeast of magnitude +4.2 phi (φ) Aquarii. This location means it peaks in altitude from London at an elevation of around 34 degrees at around 1 a.m. BST. Neptune can be seen from around 10 p.m. BST as it ascends higher in the southeastern sky.
Observers based in northern mid-latitudes have waited a very long time for Neptune to reach a maximum exploitable altitude. Indeed, since the end of World War II, Neptune has not appeared so far north of the sky. It lingered for a long time in the depths of the southern sky for most of the second half of the twentieth century. Now Neptune is about four degrees south of the celestial equator and is near the end of its seemingly endless journey to the northern sky!
Neptune takes about 165 years to orbit the Sun; in 2011, it completed its first orbit since its discovery in 1846.
A modestly sized 100-150mm (four to six inch) class telescope, operating at around 100x, is powerful enough to resolve Neptune’s tiny 2.3 arc second blue-green disc. Unfortunately, even a large telescope struggles to glean a lot of detail from its rather bland disk, although high-resolution planetary imagers should have more success, especially when using infrared filters in conjunction with a sensitive camera. infrared.
Unlike Uranus, Neptune has a giant moon, called Triton; in Greek mythology, a god of the sea and the son of Poseidon and Amphitrite. Triton is 2,710 kilometers (1,680 miles) in diameter, making it the seventh largest moon in the solar system.
Triton completes an orbit of Neptune in 5.9 days and, in our view, its unique retrograde motion along an elliptical trajectory takes it up to 17 arc seconds away from Neptune. Triton shines at a magnitude of +13.4, bright enough to be found through a 250-300mm (10-12 inch) telescope.