Mohawk Valley Astronomical Society

Return to Newsletter Index

Tropic of Canopus

by Perry Pezzolanella, MVAS

Many times while observing on a rare clear night here in Central New York we may wish we could see the great Omega Centauri globular cluster, the brilliant star Canopus, the Eta Carina Nebula, the Southern Cross, or some of the other wonders of the far southern sky that are hopelessly lost from our view here in the Northern Hemisphere. Unfortunately, there is no magical trick to make these visible in our skies, but how far south do we have to travel to see them? The answer is not always “way down south,” “Florida,” “Australia,” “the Caribbean,” “south of the equator,” or some other far distant location. It is surprising how close to home many of these gems can be seen.

How would one determine how far south one would have to travel before a far southern celestial object would appear above the southern horizon? NGC6231 is part of a beautiful grouping of stars, star clusters, and a nebula that comprises an asterism known as the “False Comet”. It is located in southern Scorpius at about -40º declination. The MVAS Apollo Observatory is almost exactly at 43º north latitude. Therefore, by taking 90º and adding it to the declination of the False Comet, -40º, this gives 50º. Then by simply subtracting our latitude of 43º from 50º one gets 7º. This means that the False Comet rises to a maximum height of 7º above the observatory’s southern horizon. Even though there are a few distant trees, it is indeed visible and a beautiful sight during summer nights.

By checking a star chart, one discovers that Omega Centauri is at declination -47º, which means that it is right on our southern horizon! The math clearly shows this: adding 90º to its declination of -47º, gives 43º. Then subtract our latititude of 43º from 43º and the answer is 0º. You could almost say that we live on the “Tropic of Omega Centauri,” in this case this imaginary line would be defined where anyone on Earth at 43º north latitude would have this beautiful globular cluster right on their southern horizon. If we traveled further north, it would dip below the horizon. Traveling further south would place it higher above the southern horizon. It is a good possibility that one would only have to travel to the Poconos or even Cherry Springs State Park in Pennsylvania to get a good view. Although still low, you could still lay claim to seeing it. Certainly a trip to Florida is not necessary, although it would be much higher above the horizon and more dramatic there.

Canopus is a brilliant winter star far below Orion at declination -52º. Simply adding 90º to this gives 38º. This means that it is on the horizon for anyone living at 38º north latitude. A trip no farther than Richmond, Virginia, will bring this dazzling beacon just into view. The “Tropic of Canopus” falls just about between Richmond and Washington, DC. The sight of this dazzling silvery star from atop the Blue Ridge Mountains on a crystal clear winter night would be spectacular.

The Eta Carina Nebula is at declination -60º, so adding 90º to -60º gives 30º. That means anyone living at 30º north latitude would have this beautiful nebula on their horizon. Now we are getting into northern Florida, the Gulf Coast and the New Orleans area, but we are still in the U.S. To see the entire Southern Cross would require a trip well into Florida to see it in its entirety, but not necessarily all the way down to the Keys. The southernmost star in the cross, Acrux, is at declination -63º, so adding 90º to -63º gives a latitude of 27º, which means that a good horizon outside the southern fringes of Tampa will bring it into full view. There is a fine line somewhere in Florida where the entire Southern Cross just balances itself nearly upright on the southern horizon without Acrux being lost.

These are fun mental exercises and, by using a star chart and a good map of the U.S. that has coordinates for latitude, it is a lot of fun to see just how far south you would have to travel within the U.S. to see a favorite southern celestial object. While many of the extreme southern objects such as the Magellanic Clouds cannot be seen from the U.S., it is surprising how many truly are visible not far from home. Omega Centauri anyone?