Neptune: Location and Orbit

Facts in Brief

Aphelion	4.54567 x 10⁹ km
Perihelion	4.44445 x 10⁹ km
Minimum Distance from Earth	4.3059 x 10⁹ km
Rotational Period (Day)	0.6713 Days (16.11 Hours)
Orbital Period (Year)	164.8 Years* (60,189 Days) * The approximation of 365.256 days per year is used.
Orbital Inclination	1.8^o
Eccentricity	0.01
Axial Tilt	29^o 36"

Relative Location

Neptune is a superior planet: its orbit lies outsides the Earth. It is normally the eighth planet from the sun, but it was actually the ninth planet from 1977 to 1999. This is due to Pluto’s highly elliptical orbit that, on occasion, passes within the more circular and regular orbit of Neptune. For the next 228 years, however, Neptune will remain our eighth planet. Neptune is more than four billion kilometers from Earth and roughly four and a half billion kilometers away from the sun.

Orbit

     Neptune has a regular orbit, just 0.01 from the circular. Only 101 million kilometers separate its points farthest and nearest from the sun, its perihelion and aphelion, respectively. This seems like a large distance, which it is—for instance, the difference is greater than Mercury’s aphelion. But considering the four-billion-kilometer distances involved, the difference is almost inconsequential.
     A day on Neptune is two-thirds that of Earth, but its year is almost 165 times longer. Given the short rotational period and the long orbital period, it should be no surprise that there are almost 89,700 Neptunian days in a Neptunian year!
     Neptune’s orbit also figures into its discovery. It passes close enough to Uranus to significantly alter that planet’s orbit, which lead mathematicians in England and France to interdependently hypothesize its existence.

Seeing Neptune from Earth

Because Neptune is so far away from the sun, only someone who knows exactly where to look can find it. Even during its closest approach to Earth, it is impossible to see the planet with the naked eye. The amateur should be prepared for a struggle if using ordinary binoculars or a telescope. Under binocular magnification, the planet appears as a faint star, identifiable though a unique greenish color. Only the largest telescopes will see it as anything other than a small disk. Because it is extremely difficult to see Neptune, even with a very powerful telescope, anyone wishing to locate it should consult a star atlas or the month’s edition of Sky and Telescope, the Royal Astronomical Society of Canada’s Observer’s Handbook, or some other astronomical calendar.

Copyright © 2000 by Gary Chan and Matthew McDermott. All rights reserved.