Philips Atlas of the Universe

Neptune

ATLAS OF THE UNIVERSE

S

ome years after Uranus was discovered, it became

clear that it was not moving as had been expected.

The logical cause was perturbation by an unknown planet

at a greater distance from the Sun. Two mathematicians,

U. J. J. Le Verrier in France and J. C. Adams in England,

independently calculated the position of the new planet,

and in 1846 Johann Galle and Heinrich D’Arrest, at the

Berlin Observatory, identified it. After some discussion it

was named Neptune, after the mythological sea-god.

Neptune is too faint to be seen with the naked eye; its

magnitude is 7.7, so that it is within binocular range.

Telescopes show it as a small, bluish disk. In size it is

almost identical with Uranus, but it is appreciably more

massive. The orbital period is almost 165 years. Like all

the giants, it is a quick spinner, with an axial rotation

period of 16 hours 7 minutes. Neptune does not share

Uranus’ unusual inclination; the axis is tilted by only

28 degrees 48 minutes to the perpendicular.

Though Uranus and Neptune are near-twins, they are

not identical. Unlike Uranus, Neptune has a strong source

of internal heat, so that the temperature at the cloud-tops is

almost the same as that of Uranus, even though Neptune

is over 1600 million kilometres (1000 million miles) fur-

ther from the Sun. In composition Neptune is presumably

dominated by planetary ‘ices’, such as water ice; there

may be a silicate core surrounded by the mantle, but it is

▼ Neptune’s clouds two

hours before Voyager 2’s

closest approach. In this

view, reminiscent of Earth

from an airliner, fluffy white

clouds are seen high above

Neptune. Cloud shadows

have not been seen on any

other planet.

 Three prominent features

reconstructed from two

Voyager images. At the north

(top) is the Great Dark Spot.

To the south is the ‘Scooter’

which rotates around the

globe faster than other

features. Still further south is

the feature called ‘Dark Spot

2’. Each moves eastwards

at a different velocity.

PLANETARY DATA – NEPTUNE

▲ Neptune’s blue-green

atmosphereseen by Voyager

at a distance of 16 million km

(10 million miles). The Great

Dark Spot at the centre is

about 13,000 
6600 km

(8000 
4100 miles). ‘Cirrus-

type’ clouds are higher.

Sidereal period 60,190.3 days

Rotation period 16h 7m

Mean orbital velocity 5.43 km/s (3.37 miles/s)

Orbital inclination 1° 45’ 19.8”

Orbital eccentricity 0.009

Apparent diameter max. 2.2”, min. 2.0”

Reciprocal mass, Sun = 1 19,300

Density, water = 1 1.77

Mass, Earth = 1 17.2

Volume, Earth = 1 57

Escape velocity 23.9 km/s (14.8 miles/s)

Surface gravity, Earth = 1 1.2

Mean surface temperature 220°C

Oblateness 0.02

Albedo 0.35

Maximum magnitude 7.7

Diameter 50,538 km (31,410 miles)

Earth

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