For summer observing, I like to travel south of the equator. That is, south of the celestial equator. There, you can find objects that would pass directly overhead for observers living in the Southern Hemisphere but are still easily visible to residents of Durango and the rest of the Four Corners.
To understand how that could be possible, it might help to think of the sky as the ancients did – that everything was literally fixed on a sphere that enclosed the Earth and rotated overhead. To know just how far south you can see, it is useful to understand coordinates on the celestial sphere. The celestial coordinates are related to coordinates on the Earth, but since the Earth is rotating, what you see overhead is constantly changing as things rise in the east and set in the west.
The coordinate that corresponds to longitude is called right ascension. For any given location on Earth, longitude is fixed in comparison to the prime meridian, which passes through Greenwich, England. Your own meridian is the imaginary north-south line of your location. Over the course of a day, the entire celestial sphere – all 24 hours of right ascension – will pass over your meridian.
The coordinate that corresponds to latitude is called declination. There is a one-to-one correspondence between latitude and declination. Like latitude, the units are degrees, minutes and seconds north and south of the celestial equator. Whatever latitude you are standing on, the same declination line will be running directly overhead, or through your zenith. Since Durango is roughly 37 degrees latitude, the 37-degree declination line will be directly overhead.
While we can’t see the Earth’s equator from here, we can certainly see the celestial equator, which is just 37 degrees to the south of your zenith. That is a bit less than four fist widths from being straight overhead and is still 53 degrees above our southern horizon. Everything between the southern horizon and that point 53 degrees above it is over the Earth’s Southern Hemisphere.
The southern sky is especially interesting in the summer because it includes objects around the center of the Milky Way galaxy. Right now, it is rising around midnight, but by the end of the month, it will be rising around 10 p.m. Some of my favorites include the Lagoon nebula (M8), the Omega (or Swan) nebula (M17) and the Eagle nebula (M16).
For a true southern sky observing challenge, Omega Centauri is by far the largest globular cluster visible anywhere. Although you will never get as good a view here as traveling south, I find catching glimpses of such targets rewarding. Look for it to top out at a mere 5 degrees above the southern horizon around 11 p.m. If you can avoid light pollution and have an unobstructed view to the south, you can see this faint fuzzy blob with your naked eye. It’s much better in binoculars and better yet through a telescope.
This monthJupiter is the brightest object in the southern evening sky. On Saturday, Saturn is rising just to the right of the waning gibbous moon. Both Saturn and Jupiter make great observing targets, since they will be rising earlier as May progresses.
Venus is now the morning star, and it reaches its westernmost elongation (farthest angular distance from the sun) on June 3. Mercury will also be a morning star, although much dimmer. It reaches its westernmost elongation May 17.
There are lots of stargazer activities in the queue for this summer. Events are scheduled at Chimney Rock National Monument, Mesa Verde National Park, Andrews Lake and elsewhere, so keep up with things via the Four Corners Stargazers Google group or Facebook page.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Email him at firstname.lastname@example.org.