Units of Time: Solar, Sidereal & Synodic

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  • 0:01 Sidereal, Solar, and Synodic
  • 1:00 Solar and Sidereal Day
  • 2:55 Sidereal and Synodic Month
  • 3:55 Sidereal and Solar…
  • 5:22 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will define solar day, sidereal day, synodic month, sidereal month, solar (tropical) year, and sidereal year, as well as tell you their exact lengths. After you've watched the video, take our brief quiz.

Sidereal, Solar, and Synodic

Astronomy is often full of some very confusing and odd terms. So is any tough subject, for that matter. Consequently, this lesson will group and define some odd-sounding terms for you with respect to the measurements of our days, months, and years and how they're related to the stars, Moon, Earth, and Sun, in order to try and make things easier.

This lesson will use three key terms throughout. We'll get to know:

  • Sidereal, which is a term that implies we're using a distant star as a reference point
  • Solar (or tropical), which is a term that means we've chosen the Sun as a reference point
  • Synodic, which is a term relating the average time between conjunctions of two celestial bodies as observed from a parent planet. In this lesson, it involves the conjunction (or meeting up) between the Sun and Moon as observed from Earth.

We'll see how this refers to units of time in astronomy, including days, months, and years.

Solar and Sidereal Day

Let's begin with the most basic unit of time in this lesson, one day. What is a day to you? Likely, you think of a day as something that's 24-hours long, the period of time between high noon today and high noon tomorrow, or the amount of time it takes the Sun to appear on the meridian again. All are correct, refer to basically the same thing, and all use the Sun as a reference point.

Thus, a solar day is the time it takes for the Earth to rotate on its axis so that the Sun is in the same position in the sky as it was the day before - 24 hours. Your watch and modern societies run on solar time. The solar day is the basis for civil timekeeping.

Contrast this with the sidereal day, the time it takes for Earth to rotate on its axis so that distant stars appear in the same position in the sky as the day before - 23 hours, 56 minutes, and 4 seconds. Astronomers often like to use a sidereal day, as opposed to a solar day, because it tells them when a certain star will return to the meridian the next night, making it easy to spot.

For us, however, it's important to note and explain the obvious. A sidereal day is shorter than a solar day. Why? This happens because the Earth orbits the Sun as it turns on its own axis. Over the course of a day, the Earth will move about one degree on its year-long orbit around the Sun. We get this by dividing 360 degrees in its orbit by about 365.25 days in a year.

This means the Sun appears to move on our sky one degree from west to east relative to the distant (or 'fixed') stars. Therefore, the Earth has to turn about four minutes more each day for one full solar day to occur. We get this number by dividing 360 degrees by 24 hours, for 15 degrees in one hour and thus about four minutes per degree.

Sidereal and Synodic Month

The next unit of time up from the day is the month. Just as per the days of our lives, there is more than one way to measure out the months of our lives.

As per the sidereal day, we can use the stars to measure the orbit of the Moon about the Earth. The sidereal month is the time it takes for the Moon to complete a full orbit around the Earth with respect to returning to the same place among the stars - 27.32 days.

But if we use the Moon's conjunction with the Sun as seen from Earth to measure the length of a month, then we get a synodic month, the amount of time it takes for the lunar phases to cycle through one time and for the Moon to return to the same position in the sky relative to the Sun - 29.53 days. The synodic month is sometimes called a lunation or lunar month.

A separate lesson on the sidereal and synodic month explains the details of why the synodic month is longer than the sidereal month.

Sidereal and Solar (Tropical) Year

Annoyingly to the uninitiated, astronomy has two ways by which to measure the length of a year as well.

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