Two kinds of year
Tropical and sidereal.
Measuring a year
A year can be measured in more than one way.
Do all ways give exactly the same length?
What this lesson covers
The idea
The tropical year is the time between successive spring equinoxes, on which the Gregorian calendar is based; the sidereal year, the time for the same stars to rise again at sunset, is about 20 minutes longer.
Measuring a year
A year can be measured in more than one way.
Do all ways give exactly the same length?
- Almost, but one is a little longer
- Exactly the same
- Totally different
Compare
Open both tabs.
Two years
The tropical year is the time between one spring equinox and the next. The Gregorian calendar is based on it. The sidereal year is the time for the same stars to rise again at sunset. It is about 20 minutes longer, so it takes a long time before the two calendars differ noticeably.
Tropical year: spring equinox to spring equinox (Gregorian calendar). Sidereal year: same stars rise again at sunset; about 20 minutes longer.
Notes
Tropical year: equinox to equinox. Sidereal year: same stars rise at sunset, ~20 minutes longer.
Check yourself
The time between successive spring equinoxes is the …
The Gregorian calendar is based on the …
By about how many minutes is the sidereal year longer than the tropical year?
Answer: 20 min
The sidereal year is longer by a mere 20 minutes.
The sidereal year is the time for …
- sidereal year. The sidereal year is measured with the stars.
- lunar year. The lunar year follows the Moon.
- tropical year — correct. Yes!
- tropical year — correct. Yes!
- sidereal year. The Gregorian calendar is based on the tropical year.
- lunar month. It is based on the tropical year.
- the Moon to be full again. That is the month.
- the same stars to rise again at sunset — correct. Yes!
- a stick's shadow to be short. The sidereal year is measured by the stars.