The Equation of Time (the difference between Greenwich Mean Time and solar time) is zero on Christmas Day. That means at 12:00 noon the Sun is due south - or at least it is at Greenwich. For us in Bolton it will be 12:10pm. So if you want to align a new mount or pier this makes it easy to find precise north-south. A vertical stick will cast a south-north shadow at this exact time making alignment a doddle.
In reality we can do it any sunny day as all planetarium programs will calculate the time the Sun transits for your location but it's more exciting on Christmas day.
Describe upcoming and future events here
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