Because of the distance of Neptune from Earth, its angular diameter only ranges from 2.2 to 2.4 arcseconds, the smallest of the Solar System planets. Its small apparent size makes it challenging to study it visually. Most telescopic data was fairly limited until the advent of Hubble Space Telescope (HST) and large ground-based telescopes with adaptive optics (AO). The first scientifically useful observation of Neptune from ground-based telescopes using adaptive optics, was commenced in 1997 from Hawaii. Neptune is currently entering its spring and summer season and has been shown to be heating up, with increased atmospheric activity and brightness as a consequence. Combined with technological advancements, ground-based telescopes with adaptive optics are recording increasingly more detailed images of this Outer Planet. Both the HST and AO telescopes on Earth has made many new discoveries within the Solar System since the mid-1990s, with a large increase in the number of known satellites and moons around the Outer Planets for example. In 2004 and 2005, five new small satellites of Neptune with diameters between 38 and 61 kilometres were discovered.
What is Neptune's angular diameter range?
2.2 to 2.4 arcseconds