Time

Equation of Time

4.5 - Understand the annual variation of the Equation of Time

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4.6 - Understand the causes of the annual variation of the Equation of Time

You can see from the diagram on the right that the times when a sundial is accurate are few in the year only four times in fact.

This is caused by the axis of the Earth (the obliquity of the ecliptic) and also Earth's orbit. Sometimes Earth is near the Sun and orbiting quickly, sometimes it is far away and orbiting slower.

The equation of time is written as the formula:

EOT =
apparent solar time – mean solar time
This is simply EOT = sundial time - clock time

You can also find out mean solar time by subtracting EOT from apparent solar time
Mean solar time =
apparent solar time – EOT

And it just keeps getting better! You can work out apparent solar time by adding EOT to mean solar time
Apparent solar time =
Mean solar time + EOT

 

Click on the EOT Calculations page to test your knowledge. There is also a data table on that page.

Careful now!

Sometimes the exam may give you a formula to help but you may have to reverse the formula around first. It is important to know each.