Measurements of Time: Sidereal, Synodic and Solar

Kelly Barkers, Artem Cheprasov
  • Author
    Kelly Barkers

    Kelly taught 7th grade science for 5 years and served as the Science Department Chair. She graduated from Cedar Crest College with a bachelor's degree in biology and a minor in health and wellness. While attending college, she was a peer tutor for Anatomy and Physiology.

  • Instructor
    Artem Cheprasov

    Artem has a doctor of veterinary medicine degree.

Learn about sidereal, synodic, and solar measurements of time. Discover the lengths and meanings of sidereal, synodic, and solar days, months, and years. Updated: 12/29/2021

Table of Contents


Sidereal, Synodic, and Solar Time Periods: How We Keep Time

Since the beginning of early civilizations, different cultures have tried to track and keep time. In order to help their society to function, many civilizations looked to the sky to create a dating system. Many systems were used over thousands of years and some were even quite accurate. For example, unlike the calendar used today, the Mayan calendar used two different years to track time. The first, called a tzolkin, was used to track days and followed constellations across the nighttime sky. The second, called a haab, was used to track different seasons by using the Sun. Since the Mayan calendar used multiple references to track time, including the Sun and other stars, their calendar was incredibly accurate.

Today, most of the modern world tracks time by using the Sun, however, astronomers have determined there are three major ways to measure time. These include:

1. Sidereal Time

2. Synodic Time

3. Solar Time

What is Sidereal Time?

One way astronomers can determine the time is through sidereal time. Sidereal time involves measuring the rotational rate of the Earth by using a distant star (not the Sun) as a reference point. Therefore, when using sidereal time, any star in the nighttime sky will move across the same point at the same time on a sidereal day. Though sidereal time can be calculated from any point on Earth, astronomers have developed a system that can be used internationally. Each sidereal day begins when the Sun is in a vernal equinox and crosses the prime meridian which is a line of longitude in London, England. The vernal equinox occurs when the Sun is exactly above the equator and both day and night are equal in length. This occurs in the northern hemisphere on March 20th or 21st and occurs in the southern hemisphere on September 22nd or 23rd.

However, any distant star can be used to calculate a sidereal day, which is the unit of time in this type of system. For example, anyone can note the time that a specific star in the sky appears directly overhead. A sidereal day is the time it takes for that exact star to appear directly overhead the following day. Sidereal time is helpful to astronomers when aiming their telescopes to a specific location in the sky. Additionally, astronomers use the sidereal time to calculate the rotational rate of other planets in the solar system.

What is Synodic Time?

Another way astronomers calculate time is by using synodic time. Synodic time measures the average time it takes for a celestial body's movement in relation to the Sun when observed from Earth. Ancient civilizations used the Moon and the Sun to create their own synodic time systems. For example, many civilizations developed calendars based on the different phases of the Moon. This type of calendar is called a lunar calendar and uses synodic time. A lunar month is the time it takes the Moon to complete one cycle of its lunar phases (New Moon, waxing crescent, first quarter, etc.). This is also known as the Moon's synodic period.

It takes about 29.5 days for the Moon the complete one cycle of its lunar phases. A new lunar month begins with a new moon.

Phases of the Moon

Astronomers can also calculate the synodic periods of other planets and celestial objects in the solar system. For example, the synodic period of Mars is 780 days which is the longest in the solar system. This is because Mars moves along its orbital path slower than Earth. For example, scientists can begin to record synodic time when the Earth and Mars are in opposition or when the Earth is located between the Sun and Mars. A year later, Earth has completed one orbit while Mars has only completed half an orbit around the Sun. It would take almost another year (a total of 780 days) for Mars and Earth to be in opposition again. Astronomers use this measurement of time to calculate a planet's synodic period.

What is Solar (or Tropical) Time?

The third way astronomers calculate time is by using the Sun as a reference point and is known as solar time. Solar time is sometimes referred to as tropical time. A solar day is an average time it takes for the Sun to reach its highest point from one day to the next. When the Sun is directly overhead, it is local noon in solar time. Most modern-day watches and clocks use solar time. Solar time can also be calculated by determining a solar year or the time it takes for two vernal equinoxes to appear. A solar year or tropical year on Earth is about 365 days. Since Earth's orbit around the Sun is 360 degrees, the Earth moves about 1 degree per day.

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  • 0:01 Sidereal, Solar, and Synodic
  • 1:00 Solar and Sidereal Day
  • 2:55 Sidereal and Synodic Month
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Sidereal Day vs. Solar Day

A sidereal day and a solar day are different since both systems measure different celestial bodies. A sidereal day is the time it takes Earth to rotate once on its axis so stars will appear in the same position of the sky. A solar day is the time it takes Earth to rotate on its axis so that the Sun is in the same position in the sky (noon to noon). Since Earth has to rotate around its own axis as well as orbiting the Sun, it takes longer for the Earth to complete one rotation in a solar day than a sidereal day. A sidereal day or actual rotation of Earth takes 23 hours, 56 minutes, and 4 seconds while a solar day is about 4 minutes longer.

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Frequently Asked Questions

What is the synodic period of the Moon?

The synodic period of the Moon is how long it takes for the Moon to complete one cycle of lunar phases. From new moon to new moon, it takes the Moon about 29.5 days to complete one synodic period.

How long is a sidereal day?

A sidereal day can be calculated using a distant star as a reference point. For example, an observer on Earth can observe the location of a star in the sky and measure the length of time it takes for the same star to appear in the same location again. The amount of time it takes for the star to reappear in the same location is a sidereal day.

What is the difference between sidereal and synodic?

Sidereal is the measurement of time using the distant stars as a reference point. Synodic is the measurement of the time it takes a celestial body to move in relation to the Sun. Commonly, the Moon's orbit around the Earth and its lunar phases are used in synodic time.

What is meant by synodic period?

Synodic period is the measurement of the time it takes a celestial body to return to the same location in relation to the Sun it orbits as observed from Earth. Synodic periods can be calculated for the Moon as well as other planets.

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