## Equation of Time

4.5 - Understand the annual variation of the Equation of Time4.6 - 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.

###### 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

###### Mix & Match

###### Links

Astronomy Notes Equation of time information

Sundials on the internet Equation of time

Ask a high energy astronomer More equation of time fun!