Automated instruments on the roof of Ladd Observatory monitored the sky during the total lunar eclipse of Sept. 27-28, 2015. First there is a wide field sky camera. It has a fish-eye lens which can capture an image of most of the sky above Providence. The second is a sky brightness meter which is used to monitor light pollution.

The full Moon is usually so bright that it overwhelms the sensitive camera causing the images to be overexposed. During the eclipse the Moon was dark enough that the only artifact in the image above is a thin vertical line where one column of the digital camera was saturated by the moonlight. The camera is more sensitive than the human eye allowing the Milky Way to be seen during the total phase of the eclipse. The time lapse video contains 3,625 still images. Each second of the movie shows about 5 minutes of changes in the sky.

The graph below shows the measurement from the sky brightness meter. It is focused on a spot about 20 degrees wide near the zenith. The time shown is UTC, the timezone of the Royal Observatory at Greenwich. It was 8pm local time at 0 hours UTC.

There is a sharp drop in sky brightness after 23 hours as twilight ends. The sky then begins to brighten as the Moon rises to a higher elevation. This increase in brightness is offset by the Moon darkening as it moves into the outer shadow of the Earth, called the penumbra. At about 1:07 the Moon begins to move into the umbra, the darker central shadow. By 2:11 the eclipse is total and lasts until 3:23. The Moon is now high above the horizon as it moves out of the umbra and the sky rapidly brightens. It is completely out of the umbra at 4:27 and a short time later exits the penumbra. The sensor detects less light overhead as the Moon begins to set. A little after 8:00 the Moon “sets” behind the dome of the observatory which suddenly blocks some of the direct light. Clouds cause a jump in brightness just before morning twilight.

For comparison, here are the measurements during a full Moon without an eclipse. The brightness of the sky measured at the zenith slowly increases and then decreases as the Moon is nearer to the field of view of the sensor.