Observation Tid-Bit #3

Observation Tid-Bit #3 - “When stars go the way of the dinosaurs.”

- by Aaron Clevenson

So far we have talked about Seeing and Transparency. Last month we mentioned Atmospheric Extinction. This month we will try to quantify its affect on stars and other celestial objects being observed close to the horizon.

Below 18 degrees altitude it is hard to accurately quantify the affects of Atmospheric Extinction on the Transparency. At this point you have so much atmosphere between you and the object that Seeing, the stability of the air, becomes a major factor in Transparency. Along with more air to be turbulent, you also have more dust, smoke, and water vapor to cause problems as well. But even above 18 degrees, Atmospheric Extinction reduces what you can see. Here are the factors to subtract:

- below 20 degrees, subtract 0.5 magnitude from the overhead Transparency,

- below 24 degrees, subtract 0.4 magnitude from the Transparency.

- below 29 degrees, subtract 0.3 magnitude from the Transparency.

- below 36 degrees, subtract 0.2 magnitude from the Transparency.

- below 52 degrees, subtract 0.1 magnitude from the Transparency.

That’s right, even at 50 degrees above the horizon you are losing 0.1 magnitude.