Neptune is currently the most distant planet from the sun, with an orbital radius of 4.5 milliard kilometers (2.8 milliard miles, or 30 Astronomical Units). Even though its diameter is about four times that of the Earth (49,420 vs. 12,742 km), ground-based telescopes reveal a tiny blue disk that subtends less than 1/1200 of a degree (2.3 arc-seconds).
Neptune has therefore been a particularly challenging object to study from the ground because its disk is badly blurred by the Earth's atmosphere.
| Please also visit my Planets |
|