Astronomy – October 2019

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Neptune

54 ASTRONOMY • OCTOBER 2019

detail-bearing images of Uranus have

come courtesy of the Hubble Space

Telescope. The visible-light images reveal

a bright south polar zone, dark clouds,

and several faint light and dark bands.

Viewed through almost any telescope,

Uranus appears as a greenish (greenish-

blue through large apertures), featureless

disk. At high power, you might detect

that it’s slightly elliptical because of its

rapid rotation. However, because the disk

is so small and most of us don’t enjoy

perfect seeing, it’s difficult to see any of

the planet’s details.

Observing Neptune

Neptune isn’t hard to find for an amateur

astronomer with a medium-size telescope.

It presents a small bluish disk that’s always

out of the range of naked eyes. Locating it

is more problematic through small optics.

If you have a 3-inch or smaller scope or

are using binoculars, first verify the plan-

et’s position against the background stars

by using the chart on page 53.

Neptune, with an average apparent

motion against the background stars of

only 22" per day, takes approximately 85

days to traverse a distance equal to the

width of the Full Moon. Currently,

Neptune is in Aquarius, the same con-

stellation it was in when Galle discovered

it. (On June 8, 2011, the planet completed

its first orbit since discovery.) Because of

the lack of nearby bright stars, Neptune’s

a bit harder to locate than Uranus. First

find magnitude 3.3 Skat (Delta [δ]

Aquarii). You’ll find the planet not quite

11° north-northeast of that star. In fact,

Neptune was less than 10' from magni-

tude 4.3 Phi (φ) Aquarii at opposition.

Observing Uranus

The visible atmosphere of Uranus is gen-

erally a featureless haze. But throughout

the planet’s history, many observers have

seen its details. The first such observer,

British astronomer William Buff ham,

noticed two round bright spots and a

bright zone in 1870. In 1883, American

astronomer Charles Augustus Young

reported markings, along with both

polar and equatorial belts. These details,

unfortunately, have one thing in com-

mon: They were exceedingly faint.

Even the Voyager 2 mission that f lew

past Uranus in January 1986 showed a

nearly featureless globe. The best recent

Date

Greatest

eastern

elongation

Sep. 6 6 P.M. EDT

Sep. 12 3 P.M. EDT

Sep. 18 12 P.M. EDT

Sep. 24 10 A.M. EDT

Oct. 6 4 A.M. EDT

Oct. 12 1 A.M. EDT

Date

Greatest

eastern

elongation

Oct. 17 10 P.M. EDT

Oct. 23 7 P.M. EDT

Oct. 29 4 P.M. EDT

Nov. 4 12 P.M. EST

Nov. 10 9 A.M. EST

Nov. 16 6 A.M. EST

TRITON

ABOVE: Neptune, named for the

Roman god of the sea, is the only

planet whose predicted position

led to its discovery. It takes 164

years and 292 days to revolve

around the Sun. Neptune has

nearly 58 times Earth’s volume

and 17 times its mass. Its largest

moon, Triton, was discovered just

17 days after Neptune. NASA

LEFT: This amateur image of

Neptune, taken October 10, 2017,

through a 12½-inch reflector and

a near-infrared filter, captured a

bright spot on the planet along

with Triton, Neptune’s largest

moon. The star to the lower right

glows at 12th magnitude in

Aquarius. MARC DELCROIX

Track Neptune’s Triton this autumn by calculating

how far it has progressed along its orbit since its

previous greatest eastern elongation.

Neptune

Triton