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A year-long keogram

Submitter: Cees Bassa
Description: The sky above our heads is always in motion, clouds passing by, the Sun and the Moon rising and setting daily and moving through the constellations at their yearly and monthly cadences. This image captures all this in the form of a year-long keogram, made from 2.1 million exposures taken with an all-sky camera, a camera that sees the entire sky, and concatenating the pixels from the column along the North-South meridian over time of day and time of year.

A keogram was originally designed to study the Northern Lights dancing accross the night sky, and is now used by amateur astronomers for their home-built all-sky cameras to show if the night sky at their location was clear.

Besides the occasional accidental power cut, my all-sky camera ran continuously during 2021, taking an image every 15 seconds, allowing me to make this keogram. It primarily shows the brightness of the sky as a function of time of day during 2021, showing how the days lengthen and the nights shorten from January to June, and reversing after the summer solstice, revealing the hourglass shape. It shows that the earliest and latest sunrise and sunset are offset by a few days from the summer and winter solstices, and shows the Sun rising higher in the sky during summer compared to winter as the bright patch around 13h local time.

The diagonal bands starting near the end of January and repeating at a roughly monthly cadence is the Moon moving accross the sky, starting in the evening sky during first quarter, being brightest in the middle of the night when it is near full Moon, and ending in the morning sky around last quarter. It is also obvious that the Moon is brighter during the winter months compared to the summer months, simply because the full Moon is higher in the sky during winter (by virtue of being opposite to the Sun).

Finally, there are lots and lots of clouds passing by, with quite a few completely overcast days -- not a surprise for a country like the Netherlands.
Copyright: Cees Bassa
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