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shadows—the shadow is shortest when the Sun is highest in the sky. This

Science natural daily cycle helps us measure time.
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However, in a lunar calendar the seasons do not stay alighed to the same

Testimony lunar months each years. This happens because a lunar year has about 354
A leap year occurs ever days, while the solar year – on which the seasons depend – is about 365
four years, with the
excep on that years days long.
divisible by 100 are not
leap years unless they are SOLAR CALENDARS
also divisible by 400.
Knowing when each season arrived was crucial, especially for agriculture.
To align the year with seasonal changes, people developed solar calendars.
The Gregorian calendar, widely used today, is a solar calendar. The months

in solar calendars are adjusted to add up to 365 days. That is why in
Gregorian calendars, some months have 30 days, others 31, and February
has only 28 days. Besides 365 days, the Earth takes nearly an extra quarter
of a day to complete one revolution around the Sun. These extra hours
together amount to nearly one full day in bout four years. To balance this

difference, solar calendars include one extra day every four years – known
as a leap year. In the Gregorian calendar, if a year divisible by four, is treated
as a leap year. So in a leap year, February has 29 days, which keeps the

calendar well synchronised with the seasons.
It takes Earth slightly under 365¼ days to travel from one spring equinox to
the next. To maintain alignment with the seasons, an extra day is added

every four years, called a leap year . However, over long periods this
addition slightly overshoots the actual time taken by Earth. To errect this
excess leap years are omitted once every hundred years—for instance in
Science 1700, 1800, and 1900. However, skipping all those would cause the
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calendar to fall behind a bit, so every 400 years, we add the leap year back
Testimony in, like in 1600 and 2000. These periodic adjustments keep the calendar
Even though leap years
keep our calnders aligned well aligned with the seasons for a long time.
with the seasons, the As we discussed earlier, Earth's revolution around the Sun – moving from
Earth's rota on is not
perfectly steady. To the spring equinox to winter equinox and back – bring about the change of
maintain precese me, seasons. The internal from one spring equinoxes to the next is called the
scien sts occasionally
add a “leap second” to tropical year. Gregorian calendar is based upon tropical year. We have also
atomic clocks. This small learnt earlier that the stars that rise at sunset change throughout the year
correc on ensures that due to the Earth's revolution around the Sun. The time taken for the same
coordinated Universal
Time (UTC) stays in sync stars to appear again at sunset is called a sidereal year , and it can also be
with the Earth's slightly used to define a solar calendar. The sidereal year exceeds the tropical year
irregular rota on.
by about 20 minutes, and so the differences between the two calendars

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