Hunt the last planet

When astronomy enthusiasts look back on 2015, the unveiling of Pluto surely will rank among the highlights. The distant world has fired the public’s imagination ever since American astronomer Clyde Tombaugh first spotted it in 1930. An intriguing and enigmatic object for most of the 85 years since, planetary scientists will get their first detailed views this July when the New Horizons spacecraft flies past.

Coincidentally, Pluto also comes to peak visibility in Earth’s sky during July. Although the dwarf planet shines feebly at magnitude 14.1, observers under a dark sky with the right equipment who know exactly where to look can glimpse the dim glow with their own eyes. Pluto reaches opposition July 6, when it lies opposite the Sun in our sky and stays visible all night. But the planet’s visibility changes so slowly that it remains a tempting target all month.

To take advantage of this Pluto viewing opportunity, you’ll want to use an 8-inch or larger telescope. Although expert observers under excellent conditions have spotted the speck of light through 5-inch scopes, the added light-gathering power of larger instruments makes the task far easier. If you don’t have a telescope big enough, consider hooking up with a member of a local astronomy club who does.

Once you’ve got your gear ready, line up a first-class observing site. For Pluto hunting, this means one that offers a dark sky as well as good seeing conditions. You’ll get steadier eyepiece views if you look out over a grassy field or a wooded expanse. Don’t aim your scope over areas that absorb the Sun’s heat in daytime and reradiate it at night, such as asphalt parking lots or your neighbor’s house.

Just as the lights of a city or town can drown out Pluto’s glow, so too can the Moon’s natural illumination. Try to observe when our satellite is out of the sky, preferably within a week or so of the July 15/16 New Moon (fittingly at the same time that New Horizons will be sending its most detailed images).

Once the night you’ve targeted for your search arrives, plan to reach your site by sunset. Set up your scope right away so it can start to cool to the air temperature. In the hour or so this process takes, your eyes will adapt to the darkness.

Sliding through the Archer
Now you are ready to search for Pluto. First, home in on a triangle of bright stars in the northeastern part of Sagittarius the Archer. Pi (π), Omicron (ο), and Xi² (ξ²) Sagittarii lie due north of the handle in that constellation’s conspicuous Teapot asterism.

Use magnitude 3.5 Xi² as an anchor to star-hop to Pluto with the help of the telescopic view above. We plotted the planet’s positions during the evening hours for North America. The chart shows background stars to magnitude 14.5, so you should be able to discern Pluto. If you can’t tell which point of light it is, sketch five or six stars near the correct position. Then return to the same field a night or two later. The “star” that moved is Pluto. Don’t expect to see the cratered landscape that New Horizons likely will reveal. Instead, simply marvel at your ability to see this dim and no longer quite so mysterious dot from across the solar system.