Thursday, January 15, 2009


The meridian is the imaginary line in the sky that runs from north to south, passing through the zenith (the point directly above your head). This construct is useful because at any given time we can then say a particular star or other celestial object is “east of the meridian” or “west of the meridian.” If it’s west of the meridian, then we know it’s moving toward the western horizon or “setting.” If it’s east of the meridian, we know it’s “rising” or moving towards the meridian.

The exceptions to this are the stars and objects of the circumpolar constellations, the ones that—instead of rising and setting—endlessly circle Polaris, the North Star, counterclockwise. We’ll take a closer look at their motions relative to the meridian another time.

When an object crosses the meridian, we say that it is culminating. We might say, for example, “such-and-such galaxy culminates at 11:00 p.m.” This tells any astronomy geeks within earshot that that’s the time they want to drag their telescopes outside to look at that object. This is because an object’s culmination is the highest altitude it can attain above the horizon, as seen from a particular observing location on Earth.

Generally speaking, the higher an object is above the horizon, the better our view of it will be. This is because Earth’s atmosphere has a distorting effect on the object’s light as it travels to our eyes. When the object’s higher in the sky, we're looking through less of that distorting atmosphere.

An object’s culmination varies according to the observer’s latitude. For example, someone observing an object from Tucson, Arizona would see it culminate higher above the horizon than someone observing in Davenport, Iowa.

Triangle as tool in Bayer's 1603 star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

Currently, about an hour after sunset, you can find one of the five smallest constellations in the northern hemisphere on or near the meridian. The pennant-shaped asterism (star pattern) at the heart of the constellation Triangulum the Triangle is a scalene triangle, having three sides of different length. Its distinctive shape has been known since antiquity, when it was associated with the fourth letter of the Greek alphabet, the triangular Delta. A number of classical star atlases depict the constellation as a drafting tool.

1) To find it, face south. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you’ll be facing approximately south.

Star maps created with Your Sky

2) In a dark site, away from bright lights, tilt your head back until you’re looking at the zenith. Triangulum will be near the zenith or slightly south of the zenith. It is east of the Great Square of Pegasus and southeast of the two star chains of Andromeda. Before hunting for it, note on the star map above how small it is compared to the Great Square.

3) The second brightest star in Triangulum marks the pointed end of the pennant and is the only one with a traditional name. This yellow-white star is called Mothallah (muh-THAH-luh), Arabic for triangle.

The brightest star, Beta, is a white subgiant surrounded by a disk of cool dust, which suggests it may have planets orbiting it. At the third point of the triangle is third brightest Gamma, a white dwarf rotating at least 100 times faster than our Sun.

4) Can you spot Triangulum Minus, a three-star asterism just south of the scalene triangle? With two sides the same length, it resembles an isosceles triangle. For over a century, Triangulum Minus was considered a constellation, but it was subsequently demoted by astronomers. These stars are pretty dim, with two of them approaching the limit of what the average person can see naked-eye, under dark skies.

Pinwheel Galaxy, M33

5) Although considered a minor constellation, Triangulum hosts one of the finest spiral galaxies in the night sky: the Pinwheel Galaxy, or M33. M33 is short for Messier (MESS-ee-yay) 33, the object’s number in the catalog of the famed 18th century French comet-hunter Charles Messier.

At a dark-sky location, you’ll be able to spot it even in binoculars, although you won’t see any detail. In a reflector telescope with six inches or more of diameter—assuming a dark location and good sky conditions—you should begin to see structure in the fuzzy mass. The more aperture (mirror diameter) you give this object, the more you’ll see.

It’s gorgeous (which, whenever possible, I like to say with a Hayley Mills/Pollyanna accent).

Astronomy Essential: Astronomy and astrology are NOT the same thing.

An astounding number of people use these terms interchangeably. Well-educated, degreed people, no less. Which is why I feel compelled to include this as an astronomy essential, to clarify the difference.

Astronomy is the scientific study of objects outside the Earth’s atmosphere. Astronomy is a science.

Astrology is the belief that celestial objects influence human beings and events, as well as the practice of divining this influence based on the positions of the celestial objects. Horoscopes, commonly seen in many newspapers and magazines, are astrological forecasts.