Thursday, October 06, 2005

TotL - Naked Eye Limiting Magnitude

All too often I hear from pedestrians the same general statement uttered as a rhetorical question: Isn't it too bright in the city to see any stars? Well, yes it is for the uninitiated and the unknowing person. An interest to look up is the first prerequisite then knowing where to look and knowing what you're looking for is next to see that the NYC sky does have stars. We often perform naked eye tests to judge transparency of our sky. This was my first lesson with learning how to see, a very important skill one can develop over time.

Personally, I concentrate on the "Corridor". That is the darkest section of sky from celestial NNW to ENE. In the Corridor and Zenith m5.2 is definitely possible, and as noted in other reports we've seen deeper on rare occasion.

The E can be washed out low and I consider the 30° deck to be the altitude which is best to look above. Despite maybe seeing m4.0+ stars, detecting DSOs may not fare as well. They'll have to be awfully bright; not hopeless, but very challenging in the least.

The SE begins to wash out and the Citicorp building has a tower of light, a hint west of celestial S that can be used to judge how hazy and humid the sky is for the night. This is used in conjunction with the SW that we've dubbed "Gegenschein". The SW is notorious for the glow that Times Square creates in our sky. Sometimes distinct beams shoot straight up nearly as high as zenith and other times it is great big wash. Nonetheless, the skyline can be stunning, and often captures the attention of pedestrians. And for this season, one can see Sagittarius's Teapot asterism poking impressing itself through the brightened glow. By conjecture, I'd say that m3.5 is possible in the SW, improving with altitude.

As for the weekend of 30 Sept. 2005, Friday & Saturday were very good for transparency and seeing. Naked eye limiting magnitude (NELM) was in excess of m5.0, maybe m5.3. Following is an outline which describes how I go about finding NELM to NE and zenith.

* Ursa Minor -- Goal count 7 stars of Little Dipper, eta UMi ~m5.0; also looks for m4.2 stars near Polaris & Kochab

* Auriga -- Leaping Minnows, 3e stars which range from m4.5 to 5.9

* Perseus -- alpha Perseid Association, typically four stars, a quadrilateral with short side close to Mirfak, between Mirfak and delta Per North of Mirfak towards the Steeple of Perseus, up to four stars reaching ~m5.2 just S of center between tau- and gamma Per.

* Taurus -- The Pleiades, typically 6 stars are seen, exceptional evenings Celaeno and Asterope are detectable

* Cassiopeia -- Cas upsilon^1 & -u ^2; HIP 4151, just NW of gamma Cas. These are ~m4.8 stars. South of Ruchbah, delta Cas