Sky & Telescope - July 2023 - 48

JULY 2023 OBSERVING
Celestial Calendar by Bob King
20h 11m
20h 10m
20h 09m
Distant Pluto Beckons
at Opposition
July is the best time to challenge yourself by trying to
catch this famous but faint dwarf planet.
N
othing screams minor planet like
having a number in front of your
name. We rarely think about it, but
Pluto's complete astronomical designation
is 134340 Pluto. Although
some still prefer to consider it a planet,
since 2006 the distant world has been
categorized as a dwarf planet. The
International Astronomical Union
(IAU) defines a dwarf planet as a " an
object in orbit around the Sun that is
large enough to pull itself into a nearly
round shape but has not been able to
clear its orbit of debris. "
When Clyde Tombaugh discovered
Pluto in 1930, it stood alone. Calling
it a planet made sense then, but
now it has plenty of company in the
outer solar system's Kuiper Belt, where
maybe up to 200 dwarf planets reside.
In similar fashion, astronomers initially
considered the first asteroids discovered
to be planets until their ranks
grew and their true natures became
apparent, placing them into a category
of their own.
Whatever your personal views are
on Pluto's standing, there's one thing
all observers can agree on: It represents
an enjoyable visual challenge. At
opposition on July 22nd (4h UT), Pluto
glimmers at magnitude 14.4 - near
the theoretical limit for an 8-inch
telescope used under a dark sky. If you
haven't already made Pluto's acquaintance,
now is the time because the icy
orb is sailing away and growing fainter
with each passing opposition. It was
48 JULY 2023 * SKY & TELESCOPE
most recently at perihelion (closest to
the Sun in its orbit) in 1989 when it
shone at magnitude 13.7, and it reaches
aphelion (greatest distance from the
Sun) in 2114, when it only musters
magnitude 16.1.
Pluto begins the month in western
Capricornus before crossing into eastern
Sagittarius on the night of July 7-8.
Its southerly declination of −23° poses
an additional challenge for observers
at mid-northern latitudes. From 40°
north, the dwarf planet stands just 27°
high when it crosses the meridian on
opposition night. Begin your hunt after
11 p.m. local daylight time when Pluto
reaches sufficient altitude for seeing
conditions to be favorable.
Pluto is currently in the middle of
nowhere with the nearest " bright " star
being 6th-magnitude SAO 188829,
about 1° to the west-northwest on
opposition night. An easier-to-identify
signpost to Pluto's field is the 8.6-magnitude
globular cluster M75. Your
quarry lies about 1° south of the cluster
in mid-July.
Use our chart at right to pin down
Pluto's position. Once you've identified
a suspect, make a quick sketch showing
its location in relation to several
nearby field stars, and then return the
next clear night to see if your suspect
has moved. If it has, congratulations
- you've sighted Pluto. Around opposition
it drifts westward 1.4′ per night,
a sufficient distance to detect at high
magnification after just 24 hours. On
the nights of July 23rd and 24th, the
dwarf planet passes about 1.5′ north of
a pair of 11th-magnitude stars separated
by 26″. They make a convenient
place to lie in wait and track Pluto's
nightly perambulations.
Once you have successfully corralled
this remote world, consider all the wonders
squeezed into that dim pinprick of
8
7
9
10
11
12
13
14
CAPRICOR
June
1
5
9
13
17
21
25
Star magnitudes
Sky & Telescope - July 2023

Table of Contents for the Digital Edition of Sky & Telescope - July 2023
