APOD: 2022 March 21 - The Sky in 2021

Good morning, Rallypoint, and welcome to the March 21st Astronomy Picture of the Day (APOD): "The Sky in 2021." Imagine a fish-eye lens on a camera focused on a single point in the sky. Now imagine a series of photos taken by that camera over the course of time during the day, and that process repeating over subsequent days. This would result in the keogram featured in today's APOD.

The word 'keogram' is based on the Inuit word for the aurora borealis: 'keoeeit.' A keogram is a method of visualizing how light changes over the course of time. The methodology was developed in the 1970s to help study the phenomena associated with auroral displays in our upper atmosphere.

Back to today's APOD. From top to bottom, you are looking at a time lapse composite image from 2021 data captured in the Netherlands. Timescale is on the bottom (x-axis) of the dataset (midnight is just to the left of center). The light blue on either side of the hour-glass shape denotes daylight versus night time (days are longer in June/July than they are in January/December). The gray in the daylight areas are times when the weather was cloudy. The twelve stripes in the hour glass were created by the passing of the Moon. You can clearly see where the equinoxes (spring, summer, fall, and winter) fall. And the seasons roll on...