Sky & Telescope - October 2020 - 51

Bag Uranus on Halloween Night
3h 00m

α

β

ARIES
Star magnitudes

+16°

2h 00m

2h 30m

γ

2

19

+20°

29

Path of Uranus

3

Path of Uranus
Aug 1

4

July 1
2020

Sept 1

5

α

μ

ξ2

CETUS

γ
M77

ξ1

δ

+10°

PSC

Star magnitudes

Mar 1

6

λ

+14°

Oct 1
5

Nov 1

6

ARIES

7

Feb 1
Dec 1
Jan 1
2021

8
31

9

α

+12°
0°

YOUR TRICK-OR-TREAT BAG could
rustle with something extra special this
Halloween. That night, Uranus comes to
opposition in a barren region of sky in
southern Aries. You can practically hear
the wind whistle through the emptiness
there and perhaps even imagine the
ghost of William Herschel, the planet's
discoverer, lifting a craggy ﬁnger to
point the way.
At magnitude 5.7, Uranus is easy to
spot in binoculars even from the suburbs. The fact that so few stars populate
the area makes this opposition ideal for
a challenge attempted by few amateurs:
ﬁnding the planet without optical aid.
From a dark, rural sky it's not too
difﬁcult to see stars slightly fainter than
Uranus. Pick a night when there's no
Moon and when the planet is near the
meridian. A particularly good opportunity comes in the third week of October
after moonset. That's when you can
make use of a fortuitous alignment.
Uranus sits roughly midway between 19
and 31 Arietis - stars with magnitudes
nearly identical to that of the planet.
Use averted vision to look for this line
of stellar equals.
Uranus currently lies 2.8 billion
kilometers from Earth. Although it's
slightly less massive than Neptune, Uranus is 1,600 kilometers wider, making

2h 20m

2h 30m

2h 40m

it the solar system's third biggest planet
after Jupiter and Saturn. Through a
6-inch telescope at 75×, Uranus betrays
its planetary nature with a telltale tiny
disk 3.8″ across.
In a 10-inch or larger instrument
under dark skies, you can pull in the
planet's two brightest moons, Titania
and Oberon (magnitudes 13.9 and
14.1, respectively). Use 250× or greater
to discern these dim specks from the

Minima of Algol

29
30

spillover glow surrounding the planet.
Our Moons of Uranus tool will help you
keep tabs on the planet's ﬁve brightest
satellites: https://is.gd/UranusMoons.
Uranus is famous for its sideways tilt
- its rotational axis is inclined 97.8° to
the plane of its orbit. It makes me think
of a dropped penny rolling away on
the pavement. Snatch up this pale blue
coin and tuck it into your treat bag on
Halloween night.

18

PERSEUS
38

Algol

21

TRIANGULUM

34

S With the end of summer in the Northern
Hemisphere, Perseus climbs in the northeastern sky in the evening. Every 2.7 days, Algol
(Beta Persei) dips from its usual magnitude 2.1
to 3.4 and back. Use this chart to estimate its
brightness in respect to comparison stars of
magnitude 2.1 (Gamma Andromedae) and 3.4
(Alpha Trianguli).

Sept.
2
5
8
11
14
16
19
22
25
28

UT
12:53
9:42
6:30
3:19
0:08
20:56
17:45
14:33
11:22
8:11

Oct.
1
4
6
9
12
15
18
21
24
27
29

UT
4:59
1:48
22:37
19:25
16:14
13:03
9:52
6:40
3:29
0:18
21:07

These geocentric predictions are from the
recent heliocentric elements Min. = JD
2445641.554 + 2.867324E, where E is any
integer. For a comparison-star chart and
more info, see skyandtelescope.org/algol.

sk yandtelescope.org * OCTOBER 2 02 0

51
https://www.is.gd/UranusMoons http://www.skyandtelescope.org/algol http://www.skyandtelescope.org
Sky & Telescope - October 2020

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