Context:
Science of leap year | The Hindu
Leap Year:
- Instead of the usual 28 days, February will have 29 days.
- The year “2024” is a leap year
Why leap year comes once every 4 years?
- Calendar year is 365 (approximation of 365 days, 5 hours, 48 minutes and 46 seconds) days long.
- Measures the time the earth takes to complete one revolution around the sun.
- Extra 5 hours, 48 minutes and 46 seconds need to be accounted for. Otherwise, the time at which different seasons start and will begin to shift.
- To avoid the shift, an extra day is added to the month of February every four years. This keeps the calendar year in sync with the sidereal year.
Key Terms:
- Sidereal time –
- Timekeeping system that’s based on the Earth’s rotation relative to the fixed stars.
- Measured based upon the diurnal motion of a star on the first point of Aries.
- Used by astronomers to locate celestial objects in the night sky.
- Sidereal day –
- The time interval between two successive transits of the first point of Aries over the same meridian. It measures the rotation of Earth relative to the stars rather than the sun.
- A sidereal day – 23 hours 56 minutes and 4.1 seconds – is the amount of time needed to complete one rotation.
- Sidereal year –
- The time taken for the sun to return to the same point in the sky relative to distant stars.
- A sidereal year – 365 days, 6 hours, 9 minutes, and 10 seconds long
- Solar Day –
- Refers to one complete rotation of Earth on its own axis relative to Sun.
- Apparent {True} Solar Day –
- The interval between two successive returns of the Sun to the local meridian.
- The length of true solar day keeps changing throughout the year due to earth’s orbit around sun is an ellipse and axial tilt of about 23.5° and remains tilted in the same direction towards the stars throughout a year.
- Mean Solar Day –
- The average of the true or apparent solar day over an entire year.
- It has 86400 seconds. Albeit, the amount of daylight varies significantly, the length of a mean solar day does not change on a seasonal basis.
- The length of the Mean Solar Day increases by 1.4 milliseconds per century.
- Solar year-
- Time taken by Earth to complete one revolution around the sun.
- Equal to 365 days, 5 hours, 48 minutes, and 46 seconds.
Source: The Hindu
Previous Year Question
Variations in the length of daytime and night time from season to season are due to
[UPSC Civil Services Exam – 2013 Prelims]
(a) the earth’s rotation on its axis
(b) the earth’s revolution round the sun in an elliptical manner
(c) latitudinal position of the place
(d) revolution of the earth on tilted axis
Answer: (d)
