SKY_September2014.pdf

SkyandTelescope.com September 2014 51

Learn how to use detailed star charts to fi nd the faintest things with your telescope: skypub.com/charts. Alan MacRobert

Jun 1, 2014

Jul 1

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Jan 1, 2015

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δ

ε

80

PISCES

Path of Uranus

+8°

1 h 00 m 0 h 50 m

+6°

+4°

Star magnitudes

5

4

6

7

8

9

Jun 1, 2014

Jul 1

Aug 1

Sep 1

Oct 1

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Jan 1, 2015

Feb 1

θ

ρ

σ

AQUARIUS

Path of Neptune

–8°

22 h 30 m 22 h 20 m

–10°

Star magnitudes

5

4

6

7

8

9

SkyandTelescope.com September 201 4 51

Facing page: South of the Great Square of Pegasus, 6th-magni-

tude Uranus and 8th-magnitude Neptune mosey through their

little retrograde loops. Neptune is at opposition on August 29th,

Uranus on October 7th. The closeup charts on this page pinpoint

their locations among fainter stars. Track their motion!

Uranus spends the rest of this year at about magnitude

5.7, making it a piece of cake to spot in binoculars once

you locate its position. Neptune is more challenging in

binoculars at about magnitude 7.8.

In a telescope at high power, both show tiny, fuzzy,

nonstellar disks. Uranus is currently 3.7′′ wide; Neptune

is 2.4′′. In my 6- and 12.5-inch scopes, I pretty consistently

see a trace of aquamarine blue-green in Uranus’s pale

light. Neptune looks duskier gray with a just a hint of

blue. Long-term photometry at Lowell Observatory has

shown that their colors shift slightly from time to time.

For more on their sometimes controversial colors, see the

September 2010 issue, page 56.

Unique in the solar system, these two intermediate-

mass planets consist mostly of volatile hydrogen com-

pounds: water (OH 2 ), ammonia (NH 3 ), and methane

(CH 4 ) — as opposed to the rock and iron of the terrestrial

planets and the hydrogen and helium of Jupiter and

Saturn. Planetary scientists call these and certain other

volatile compounds (such as CO 2 and CO) “ices” — even

if they only turn solid in unearthly cold, and even though

they’re actually fl uids at the high temperatures and pres-

sures of deep planetary interiors. Uranus and Neptune

are thus called the solar system’s “ice giants.”

Another thing Uranus and Neptune have in com-

mon is that they’re the solar system’s only major planets

(by the current defi nition) that were discovered rather

than always being obvious. William Herschel in Eng-

land swept up Uranus by accident in 1781 while using a

homemade refl ector in his back garden. Johann Galle at

Berlin Observatory discovered Neptune in a very deliber-

ate search in 1846, after Urbain Le Verrier predicted its

position from unexplained gravitational tugs on Uranus.

be Titan, magnitude 8.9 and farther south as shown at right.

Where the occultation happens, the combined light of Rhea

and the star (they’ll have merged into one blended image in a

telescope) will drop by 2.5 magnitudes. That’s a dramatic,

factor-of-10 decrease in brightness.

The occultation will last for up to 58 seconds, depending on

how close you are to the midline of the shadow path.

There’s no particular scientifi c gain to be had from timing

this occultation, since Rhea’s size, shape, and orbit have been

refi ned to exquisite precision from 10 years of observations by

NASA’s Cassini spacecraft orbiting Saturn. But it will certainly

be interesting to watch and perhaps videorecord.

S&T:

LEAH TISCIONE

SAO

159034

and Rhea

Enceladus

Tethys

Dione

Titan

Saturn

Sept. 13, 2014

0:38 UT

NORTH

EAST

Early on the evening of September 12th, telescope

users in eastern North America will see Saturn’s

moon Rhea, magnitude 10.2, blot out or skim just

past a 7.8-magnitude star. North is up.

CC layout.indd 51 6/23/14 12:17 PM