1) A solar day is the time it takes a planet to complete one rotation relative to the star it orbits. On Earth, a solar day is approximately 24 hours. 2) A sidereal day is the time it takes Earth to complete one rotation relative to the background stars. A sidereal day is about 4 minutes shorter than a solar day. 3) The difference between solar and sidereal days causes different constellations to be visible at different times of the year.

1. Solar Day & Sidereal
Day
By S.SRIKUMAR X - A

2. Solar Day
● The period for a celestial object to rotate once in relation to the star it is orbiting is
called Solar Day.
● It is the basis of solar time.
● It is also known as a Synodic Day.
● The time taken to complete 1 Solar Day is called Synodic Rotation Period.

3. Earth’s Solar Day
● The time it takes for the Sun to pass over the same meridian is called Earth’s
Solar (or) Synodic Day.
● 1 Solar Day on Earth has a time duration of 24 hours.
● This time duration is not constant and changes over the course of the year due
to the eccentricity of the Earth’s orbit around the Sun & the axial tilt of the
Earth.

4. ● The longest & shortest synodic days’ durations differ by about 51 seconds.
● As viewed from the Earth during the year, the Sun appears to slowly drift
along an imaginary path coplanar with Earth’s orbit, known as the ecliptic,
on a spherical background of seemingly fixed stars.
● Each synodic day, this gradual motion is a little less than 1° eastward (360°
per 365.25 days), in a manner known as prograde motion.

5. ● Certain orbits & sun-synchronous orbits have orbital periods that are a
fraction of a synodic day.
● Combined with a nodal precession, this allows them to always pass over a
location on Earth’s surface at the same mean solar time.

6. A Synodic Day In Other Planets
● Due to tidal locking with the Earth, The Moon’s Synodic Day (or) Lunar Day is the
same as its synodic period with Earth & the Sun.
● Due to the slow rotational speed & retrograde motion of Venus, its synodic rotation
period of 117 Earth Days is about half the length of its orbital period & sidereal
rotational period.
● Due to the slow rotational speed & orbit around the Sun of Mercury, its synodic
rotation period of 176 Earth Days is 3 times longer than its sidereal rotational
period and twice as long as its orbital period.

8. Sidereal Day
● Sidereal time is a timekeeping system that astronomers use to locate celestial
objects.
● Using sidereal time, it is possible to easily point a telescope to the proper
coordinates in the night sky.
● Viewed from the same location, a star seen at one position in the sky will be seen at
the same position on another night at the same sidereal time.

9. ● More exactly, sidereal time is the angle measured along the celestial
equator from the observer’s meridian to the great circle that passes
through the March Equinox & both equinoxes, and is usually expressed in
hours, minutes & seconds.
● 1 Sidereal Day on Earth has a time duration of 23 hours, 56 minutes &
4.0905 seconds.

10. ● Because Earth orbits the Sun once a year, the sidereal time at any given place & time
will gain about 4 minutes against every local civil time, every 24 hours, until, after a
year has passed, 1 additional sidereal day has elapsed compared to the number of
solar days that have gone by.
● The sidereal day is the time it takes for the Earth to complete 1 rotation about its
axis with respect to the stars.
● A measurement of the sidereal day is made by noting the time at which a particular
star passes the celestial meridian(i.e., directly overhead) on 2 successive nights.

11. Fun Fact
Q : Why are different constellations visible at only specific times of the year?
ANS : Because as we can see, our usual definition of an Earth Day is 24 hours,
so the sidereal day is 4 minutes faster. This means that a particular star will
rise 4 minutes earlier every night, and this is the reason why different
constellations are only visible at only specific times of the year.