When to measure the equation of time<\/strong><\/h2>\nThe mean and apparent Sun align on four dates throughout the year: on 15 April, then 13 June, 1 September and 25 December (correct for J2000.0).<\/p>\n
At such times, the end of the shadow of our sundial stick (gnomon) marks the correct time. On other days the mean Sun is either ahead of or behind the apparent Sun.<\/p>\n
The difference is known by the rather enigmatic name of \u2018the equation of time\u2019 and can range from 14 minutes and 15 seconds ahead of clock time to 16 minutes and 25 seconds behind.<\/p>\n
The variation can be demonstrated to great effect by photographing the position of the Sun over the course of a year at the same UT time.<\/span><\/p>\nIf you do this, the equation of time causes it to oscillate back and forth. However, Earth\u2019s axial tilt also plays a part, causing the Sun to appear to move up and down in the sky over the course of a year.<\/span><\/p>\nAs a consequence, the equation of time plus declination drift produces a bowling pin shape in the sky, known as an analemic curve. <\/span><\/p>\nMarking the position of a gnomon\u2019s shadow tip at the same time of day throughout the year will also produce this curve, and that\u2019s your challenge for the rest of 2023 and into 2024!<\/p>\n
This requires good placement of the gnomon so it\u2019ll be in Sun throughout the year.<\/p>\n
It also requires good weather. Don\u2019t worry about missing days though \u2013 the shape can still be revealed, even with many gaps.<\/p>\n
This guide originally appeared in the April 2023 issue of <\/strong><\/em>BBC Sky at Night Magazine<\/strong>.<\/strong><\/em><\/p> <\/body><\/html>\n
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