Author: Michael Richmond Date: 961007 Revision: #1 961007 Key Words: techniques Galaxy
On Sep 29, 1996, I acquired some TASS images with the Vermont triplet. The weather had been unsettled, and the forecast called for rain late in the night (which did occur). I resigned myself to taking a few hours of scans just after dark, and decided that one good use for the data would be to study the variation in sky brightness caused by sunset and moonrise. The gibbous moon was scheduled to rise about 3 hours after sunset.
So, I got the scans, noticing that the sky became very dark for just an hour or so after the sunlight faded -- I was able to see the Milky Way very easily -- and then brightened again as the Moon rose in the east.
After subtracting a dark vector from the data, I measured the sky value as a function of time in the following way: for each row (line) of data, I calculated the median pixel value. I then made a large file consisting of row number (from 0, soon after sunset, to about 13000, around 10:15 PM) and median sky value.
I expected to see the sky value drop steeply after sunset, reach some minimum level, and then rise again as the Moon became visible. I therefore made a plot which shows the sky value (as a function of row number) and also the altitudes of the Sun and Moon above (or below) the horizon. Astronomical twilight is said to start when the Sun is 18 degrees below the horizon: at that point, the sky is supposed to be as dark as it ever gets. I also noted that a ridge of hills to the East of the site would block the Moon for some time after it had risen above the theoretical, perfect horizon, and watched carefully to note the actual time that it appeared above the ridge.
Here's a graph of the sky value, and the solar and lunar altitudes (you can click on it to get a larger version of the figure):
The filled circle in the lower panel shows the Sun's altitude, and the open circle the Moon's altitude.
What a surprise! Note how the sky value, after dropping rapidly as the Sun sinks farther and farther below the horizon, rises slowly, stay high for about an hour, and then starts to drop as the Moon is rising into the sky!. It isn't until near the end of my scan that the moonlight (and, probably, some clouds) seems to drive the sky to larger values.
At first, I figured that this strange behavior of the sky brightness must have been due to thin clouds that I couldn't see with my eyes, but then I decided to look at the images themselves. Here's a picture of the scan from rows 3000 to 4000, roughly (again, click to get a larger version); North is up, and East to the left.
By golly, this looks like part of the Milky Way! And so it is -- a section about 7 degrees wide and 3 degrees high, centered at (1950) RA = 18:47, Dec = +0:30 -- in the western edge of Aquila. Note the very bright clump of stars near bottom left, and the strong obscuration running like a snake just above it and down to the right.
And this is the answer to my perplexity --- the reason that the sky got brighter quickly, then fainter as the moon rose, was because the camera happened to be scanning through the center of the Milky Way at that time!
Let this be a lesson to people (like myself) who try to analyze their results before they look at them.