# What is a solar day?

Oct 8, 2017

A solar day is the period between successive solar noons.

#### Explanation:

The problem with defining the length of a day is that there are several definitions.

The length of a day as measured by clocks is the mean solar day. It is exactly 24 hours long.

The sidereal day is the time it takes for the Earth to complete one rotation with respect to the fixed stars. It is about 23 hours 56 minutes.

The solar day is the period between two successive high noons. High noon being the moment the Sun is at its highest in the sky. It varies continuously from day to day due to the Earth's orbit being elliptical and due to the ${23}^{\circ}$ axial tilt. Successive days vary in length by up to 20-30 seconds.

Solar days were traditionally measured using sun dials. When clocks were invented a correction factor needed to be added to sundial time to make it agree with clocks. The time difference is called the equation of time and the difference between clock noon and solar noon can be as much as 18 minutes.

The graph shows the equation of time. The orange curve is the difference due to the Earth's orbital eccentricity. The green curve shows the difference due to the axial tilt. The blue curve is the sum of the other two curves which is the equation of time.