Equation of Time

Demonstrate an understanding of the term 'equation of time' (apparent solar time - mean solar time) and perform simple calculations

You can see from the diagram on the right that the times when a sundial is accurate are few in the year. 4 times in fact.

This is caused by the axis of the Earth (which gives us our seasons) and also Earth's orbit. Sometimes Earth is near the Sun and orbiting quickly, sometimes it is far away and orbiting slower.

There will be questions about the Equation of Time (EOT) in the exam. Click on the EOT Calculations page to attempt some. There is also a data table on that page.

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

 

Careful now!

Sometimes the exam may give you a formula to 'help' but another is needed. It is important to revise these carefully.

 

Summary

EOT =
apparent solar time – mean solar time

Mean solar time =
apparent solar time – EOT

Apparent solar time =
Mean solar time + EOT

Questions

What 2 factors cause variations in the equation of time?

Animation
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Links

Astronomy Notes Equation of time information

Sundials on the internet Equation of time

Ask a high energy astronomer More equation of time fun!