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Movement of Earth: Synodic Day and Sidereal Day

Class 7
Jun 12, 2023


The seasonal changes and other variations are not due to the elliptical orbit of the Earth. Seasonal variations result from the tilt of the Earth, whereas daily variations in light and temperature are caused by its rotation. Earth’s tilt changes the length of the days and nights during different seasons.

The Earth’s one full rotation on its axis lasts for 24 hours. The duration of that rotation is called one day. During this rotation, the side of the Earth facing the Sun is always in the light (daytime). It is dark (nighttime) on the side that faces away from the Sun.

Earth moves around the Sun as it rotates around its axis. It is known as the Earth’s revolution.

The different cycles like the day-night tides and seasonal changes we notice result from this rotation and revolution of the Earth. Earth’s revolution around the Sun takes 365 days. That is equal to one year.



Every day, we see that the Sun appears to rise from the east, move in a clockwise arc across the sky, then set toward the west and vanish for a period that we refer to as nighttime.

The Sun traveling from east to west across the sky
The Sun traveling from east to west across the sky

It appears that way because of the Earth’s rotation, as Earth rotates counter-clockwise. Periodically, this cycle—which lasts for 24 hours according to our clock—repeats. The spherical Earth rotates on its own axis every day, causing this cycle of day and night.

We can study the position of the Sun, Earth, and stars by studying the celestial sphere.

The celestial sphere is an imaginary sphere of an arbitrarily large radius, concentric with Earth. All objects in the sky can be projected inside the surface of the celestial sphere as if it were the underside of a dome.

Celestial sphere

Each day the Sun appears to move about a degree anticlockwise (eastwards) concerning the fixed stars. Along a path known as the ecliptic.

The ecliptic is known as a great circle on the celestial sphere representing the Sun’s apparent path during the year.

Synodic Day and Sidereal Day

A synodic day —or just “day”— is the period it takes for a planet (like the Earth) to rotate once in relation to the star it is orbiting (like the Sun). For Earth, the length of a synodic day is 24 hours.

The period of revolution of the Earth with respect to the Sun is called the Synodic period. When it comes to a Synodic day, a Synodic day refers to the rotation of the Earth once a day in relation to the Sun.

You may think that it means the Earth rotates only 360 degrees. However, that is not the case. As Earth is also continuously moving around the Sun, the Earth rotates a little more than 360 degrees to have the Sun at the observer’s meridian. Synodic Day is also known as Solar Day.

In 24 hours, the Earth makes slightly more than one full rotation due to its motion around the Sun. We call it one day or one synodic day.

Solar day and sidereal day

On the other hand, a sidereal day is the period of one rotation relative to distant stars.

Sidereal is related to the stars, while Synodic is related to the Sun.

A synodic day is distinguished from a sidereal day, which is one complete rotation in relation to distant stars. A synodic day is from “sunrise to sunrise,” whereas a sidereal day is from one rise of a given star reference to the next.

The length of a synodic day and the sidereal day are different due to Earth’s motion around the Sun. The position of the Sun shifts about 1° every day relative to distant stars. This fact is reflected in the difference in the length between a synodic day and a sidereal day.

Solar day and sidereal day
Solar Day and sidereal day


  • A synodic day is what we call “day” in everyday language. We divide one synodic day into 24 hours because a synodic day is slightly more than one full Earth’s rotation.
  • A sidereal day is slightly shorter than 24 hours. A sidereal day is 23 hours, 56 minutes, and 4.0905 seconds. The difference between a sidereal day and a synodic day sums up to a single day in a year.

The Earth’s orbital period is 365.242199 days. Our solar calendar of 365 days keeps pace by adding a leap-year day every four years (except once every four centuries).

Synodic Day and Sidereal Day


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