Seeing Pluto with the ASN

Everyone knows something about Pluto, including most third graders.  Yet few people have actually seen it, including most astronomers.  It’s natural to think that we in the ASN would have seen all the planets, even famous former planets.  So how come so few of us have seen Pluto?

A telescope at least 12 inches in diameter is recommended for Pluto.  Most of us don’t have a scope that big.  And there are other reasons we don’t look for Pluto: first, it’s technically difficult to identify, even with a go-to telescope (just ask the RECON folks).  Second, it’s not a very impressive sight.  In fact it looks like a very small faint star, seemingly ordinary.  You can’t recognize Pluto by just looking at it; instead you identify it by recognizing the pattern of the star field around Pluto.  In these respects Pluto is unlike most of the objects we usually find with our telescopes.  The value of the effort is in the satisfaction of knowing what you are looking at (just as with most other objects in the sky!).  Anyone who looks for Pluto needs determination and imagination. 

Since Pluto currently has a low declination, it appears only in the southern sky and only for a few months each year.  Fortunately for us, those months happen to be in the summer.  This year Pluto comes to opposition on July first.  Each summer for the next several years there will be opportunities to see Pluto, but as the years go by it will get fainter and move farther south.  Its southern location and drift means that the viewing “window” when Pluto can be seen at its highest in a dark sky is getting shorter with each passing year.    

 

To me, the challenge and satisfaction of seeing Pluto were well worth the effort.  If that’s not enough, consider that the New Horizons probe will arrive at Pluto in just two years.  This will generate a lot of public interest, and those of us who have seen Pluto will be able to describe our first-hand experience to others.  I think we in the ASN should make it a goal for as many members as possible to see Pluto.

There are two viewing windows left this year.  These windows correspond to the times and dates when Pluto is highest (due south) and the Moon is below the horizon.  They are: 1) the nights of July 1 through 14 and 2) the nights of July 28 through August 10.  Each window can last from 2 to 4 hours per night, since Pluto can be observed before or after the time of “transit” (i.e., when it is due south).

Pluto’s declination is about -20 degrees this year, so its maximum altitude in Reno will be less than 31 degrees.  At one hour on either side of its “transit” time, Pluto is lower by about 1.7 degrees of altitude.  Two hours before or after the time of transit, Pluto is more than six degrees lower than its maximum altitude (and only 24.3 degrees above the horizon).  At this altitude, atmospheric extinction will diminish Pluto’s light by about half a magnitude, even with very good sky conditions.

My recommendation is to observe Pluto within 2 hours of its transit time, provided the Moon is below the horizon.  A clear, dark sky is essential.  As a planning tool I’ve computed the following table that compares transit times to Moon rise and set times.  The table shows when Pluto is at its best in each of the two viewing windows.  All times and dates are PDT, computed for Reno:

Evening of:

Time of Pluto transit

Time of Moon rise/set

Moon rise /set

July 1

00:57

01:42

MR

July 8

00:33

20:36

MS

July 14

00:05

23:43

MS

 

 

 

 

July 28

23:12

23:43

MR

Aug 3

22:48

04:08

MR

Aug 10

22:20

21:47

MS

 

Directions for observing Pluto along with the all-important finder chart can be found on pages 52 and 53 of the June issue of Sky&Telescope.  As with the RECON program, specific methods are needed and the go-to step is only the beginning.  Pluto is currently seen in front of a dense star field, and the larger your telescope is, the more stars (i.e., fainter stars) will be seen in the background.  This can be confusing: our 20 inch scope will show stars that are fainter than the ones on the finder chart.  To know which dot is Pluto, you need to match the finder chart to the view in the eyepiece.  This is where it’s better to be part of a group like the ASN.  We can help each other out and more of us will see Pluto. 

The finder chart shows that on the evening of July 14, Pluto will pass about 3 arc minutes south of the faint (m 11) globular Palomar 8.  It should be close to Pal 8 on the evening of July 13 also.  This is a rare opportunity to have a distinct marker near Pluto.

All we have to do is decide to take on the challenge the next time we have a dark-sky star party on one of the dates in the viewing windows!

Category: Programs Committee