A Purely Lunar Calendar Slides Its Seasons Out of Place
Learn how the day, month and year come from real motions, measure a solar day by shadows, compare lunar, solar and luni-solar calendars, see why leap years exist, and link festivals to the sky.
Why would a calendar based only on the Moon drift out of the seasons?
Because twelve lunar months come to roughly 354 days, about eleven days short of the year the seasons follow. Every year the whole calendar slips earlier, so a harvest month eventually lands in the middle of winter.
The Moon and the seasons simply do not divide into each other neatly. This page covers everything in the CBSE Class 8 Science chapter's second part: defining the day, month and year, comparing calendars, leap years, and festivals and satellites.
The Moon and the seasons simply do not divide into each other neatly. This page covers everything in the CBSE Class 8 Science chapter's second part: defining the day, month and year, comparing calendars, leap years, and festivals and satellites.
How are the day, the month and the year defined?
Each comes from a real motion in the sky.
- A day is the time the Earth takes to rotate once on its axis — giving us day and night.
- A month comes from the Moon's orbit around the Earth. One complete cycle of phases, from new Moon to new Moon, takes about 29 and a half days and is a lunar month.
- A year is the time the Earth takes to revolve once around the Sun, about 365 and a quarter days, and it is what brings the seasons back around.
Measuring a solar day with shadows. A solar day is the interval between one local noon and the next, and shadows locate noon precisely:
1. Fix a straight stick, a gnomon, upright on level ground in an open space.
2. Mark the tip of its shadow every fifteen minutes through the day.
3. The shadow shortens through the morning and lengthens through the afternoon.
4. The moment the shadow is shortest is local noon, and it points along the north-south line.
5. The time from one shortest shadow to the next is one solar day.
A sundial works on exactly this principle, and old observatories used large stone gnomons for it.
The measurement is why the day and year do not fit together tidily. The Earth's rotation and its revolution are independent motions, so a year contains about 365 rotations and a quarter — a leftover that the next sections have to deal with.
- A day is the time the Earth takes to rotate once on its axis — giving us day and night.
- A month comes from the Moon's orbit around the Earth. One complete cycle of phases, from new Moon to new Moon, takes about 29 and a half days and is a lunar month.
- A year is the time the Earth takes to revolve once around the Sun, about 365 and a quarter days, and it is what brings the seasons back around.
Measuring a solar day with shadows. A solar day is the interval between one local noon and the next, and shadows locate noon precisely:
1. Fix a straight stick, a gnomon, upright on level ground in an open space.
2. Mark the tip of its shadow every fifteen minutes through the day.
3. The shadow shortens through the morning and lengthens through the afternoon.
4. The moment the shadow is shortest is local noon, and it points along the north-south line.
5. The time from one shortest shadow to the next is one solar day.
A sundial works on exactly this principle, and old observatories used large stone gnomons for it.
The measurement is why the day and year do not fit together tidily. The Earth's rotation and its revolution are independent motions, so a year contains about 365 rotations and a quarter — a leftover that the next sections have to deal with.
How do lunar, solar and luni-solar calendars differ?
Each is built on a different motion, so each keeps track of something different.
Lunar calendar — months follow the phases of the Moon. Each month begins at a new Moon and runs about 29 or 30 days, so twelve of them make about 354 days.
- Advantage: the date always tells you the Moon's phase, which is useful for tides and for festivals tied to the Moon.
- Problem: it is about 11 days shorter than the seasonal year, so it drifts.
Solar calendar — the year follows the Earth's revolution around the Sun, about 365 and a quarter days, and the months are simply convenient divisions of it. The Gregorian calendar used worldwide is solar.
- Advantage: the seasons stay fixed, so a given date always falls in the same season.
- Problem: the date tells you nothing about the Moon's phase.
Luni-solar calendar — follows the Moon for months and the Sun for the year, keeping both in step. Most traditional Indian calendars work this way.
- It corrects the 11-day gap by inserting an extra month every few years, called adhik maas.
- So the months stay tied to the Moon and the festivals stay in their proper seasons.
That extra month is the whole solution, and it explains something familiar. Indian festivals fall on different Gregorian dates each year — because they follow the Moon — but they never drift out of their season, because the added month pulls the calendar back in line with the Sun.
Lunar calendar — months follow the phases of the Moon. Each month begins at a new Moon and runs about 29 or 30 days, so twelve of them make about 354 days.
- Advantage: the date always tells you the Moon's phase, which is useful for tides and for festivals tied to the Moon.
- Problem: it is about 11 days shorter than the seasonal year, so it drifts.
Solar calendar — the year follows the Earth's revolution around the Sun, about 365 and a quarter days, and the months are simply convenient divisions of it. The Gregorian calendar used worldwide is solar.
- Advantage: the seasons stay fixed, so a given date always falls in the same season.
- Problem: the date tells you nothing about the Moon's phase.
Luni-solar calendar — follows the Moon for months and the Sun for the year, keeping both in step. Most traditional Indian calendars work this way.
- It corrects the 11-day gap by inserting an extra month every few years, called adhik maas.
- So the months stay tied to the Moon and the festivals stay in their proper seasons.
That extra month is the whole solution, and it explains something familiar. Indian festivals fall on different Gregorian dates each year — because they follow the Moon — but they never drift out of their season, because the added month pulls the calendar back in line with the Sun.
Why are leap years needed?
Because a year is not a whole number of days. The Earth takes about 365 and a quarter days to go round the Sun, and a calendar can only count whole days.
Ignore that quarter and the calendar loses about a quarter of a day every year. After four years it is a full day out of step, and over a long stretch the seasons would wander away from their dates.
The fix is the leap year:
- An ordinary year has 365 days.
- A leap year has 366 days, with the extra day added to February, giving it 29 days.
- A leap year comes once every four years, which restores the four missing quarter-days as one whole day.
- A year is a leap year if it is divisible by 4, with an adjustment for century years, which keeps the match even closer over long periods.
The Indian National Calendar is the country's official civil calendar alongside the Gregorian one. Its features:
- It is a solar calendar, based on the Saka era
- Its first month is Chaitra, and its new year begins around the spring equinox in late March
- Chaitra has 30 days in an ordinary year and 31 in a leap year
- It is used for official purposes, in government publications and by All India Radio alongside the Gregorian date
So the leap day is not an arbitrary addition. It is the calendar being corrected to match the sky — and without it, over a long enough stretch of time, the months would slide into the wrong seasons entirely.
Ignore that quarter and the calendar loses about a quarter of a day every year. After four years it is a full day out of step, and over a long stretch the seasons would wander away from their dates.
The fix is the leap year:
- An ordinary year has 365 days.
- A leap year has 366 days, with the extra day added to February, giving it 29 days.
- A leap year comes once every four years, which restores the four missing quarter-days as one whole day.
- A year is a leap year if it is divisible by 4, with an adjustment for century years, which keeps the match even closer over long periods.
The Indian National Calendar is the country's official civil calendar alongside the Gregorian one. Its features:
- It is a solar calendar, based on the Saka era
- Its first month is Chaitra, and its new year begins around the spring equinox in late March
- Chaitra has 30 days in an ordinary year and 31 in a leap year
- It is used for official purposes, in government publications and by All India Radio alongside the Gregorian date
So the leap day is not an arbitrary addition. It is the calendar being corrected to match the sky — and without it, over a long enough stretch of time, the months would slide into the wrong seasons entirely.
How are Indian festivals linked to events in the sky?
Many are fixed by the Moon's phase, and others by the Sun's position.
Tied to Moon phases:
- Diwali falls on the new Moon (amavasya) of its month, which is why it is the darkest night and is lit with lamps.
- Holi, Buddha Purnima, Guru Purnima and Raksha Bandhan fall on a full Moon (purnima).
- Eid is declared on the sighting of the crescent Moon, since the Islamic calendar is lunar.
- Karwa Chauth and Ganesh Chaturthi are fixed by named days of a fortnight.
Tied to the Sun's position:
- Makar Sankranti and Pongal mark the Sun's northward turn after the winter solstice.
- Baisakhi, Bihu and Puthandu fall around the spring equinox period, at the start of the solar year.
- The equinoxes are the two days when day and night are of equal length; the solstices are the longest and shortest days.
This is exactly why some festivals shift across the Gregorian calendar while others barely move. Moon-based festivals wander by days from year to year; Sun-based ones such as Makar Sankranti stay on almost the same date.
Artificial satellites extend this sky-watching with technology. They are launched to orbit the Earth and are used for:
- Weather forecasting and tracking cyclones
- Communication — television, telephone and internet links
- Navigation, as in satellite positioning systems
- Remote sensing — surveying crops, forests, water and minerals
- Astronomical observation from above the atmosphere
A satellite stays in orbit because it is moving fast enough sideways that it keeps falling around the Earth rather than into it — which is why it needs a rocket to reach that speed but no engine to stay there.
Tied to Moon phases:
- Diwali falls on the new Moon (amavasya) of its month, which is why it is the darkest night and is lit with lamps.
- Holi, Buddha Purnima, Guru Purnima and Raksha Bandhan fall on a full Moon (purnima).
- Eid is declared on the sighting of the crescent Moon, since the Islamic calendar is lunar.
- Karwa Chauth and Ganesh Chaturthi are fixed by named days of a fortnight.
Tied to the Sun's position:
- Makar Sankranti and Pongal mark the Sun's northward turn after the winter solstice.
- Baisakhi, Bihu and Puthandu fall around the spring equinox period, at the start of the solar year.
- The equinoxes are the two days when day and night are of equal length; the solstices are the longest and shortest days.
This is exactly why some festivals shift across the Gregorian calendar while others barely move. Moon-based festivals wander by days from year to year; Sun-based ones such as Makar Sankranti stay on almost the same date.
Artificial satellites extend this sky-watching with technology. They are launched to orbit the Earth and are used for:
- Weather forecasting and tracking cyclones
- Communication — television, telephone and internet links
- Navigation, as in satellite positioning systems
- Remote sensing — surveying crops, forests, water and minerals
- Astronomical observation from above the atmosphere
A satellite stays in orbit because it is moving fast enough sideways that it keeps falling around the Earth rather than into it — which is why it needs a rocket to reach that speed but no engine to stay there.
Exam tip
Exam tip: naming the motion behind each unit of time
This chapter is marked on which motion defines what, so state it every time.
Write the pairs explicitly: a day is one rotation of the Earth; a month comes from the Moon's orbit; a year is one revolution of the Earth around the Sun. Mixing rotation and revolution is the commonest error.
Quote the figures — about 29 and a half days for a lunar month, about 365 and a quarter days for a year, and the 11-day shortfall of twelve lunar months.
For leap years, give the reason (the quarter-day remainder) and the rule (one day added every four years, in February).
For the shadow experiment, say that noon is when the shadow is shortest, and that a solar day is the interval between successive noons.
And for a festival question, name whether it follows the Moon or the Sun, since that is what the mark is for.
Write the pairs explicitly: a day is one rotation of the Earth; a month comes from the Moon's orbit; a year is one revolution of the Earth around the Sun. Mixing rotation and revolution is the commonest error.
Quote the figures — about 29 and a half days for a lunar month, about 365 and a quarter days for a year, and the 11-day shortfall of twelve lunar months.
For leap years, give the reason (the quarter-day remainder) and the rule (one day added every four years, in February).
For the shadow experiment, say that noon is when the shadow is shortest, and that a solar day is the interval between successive noons.
And for a festival question, name whether it follows the Moon or the Sun, since that is what the mark is for.
Did you know
Why does Diwali fall on the darkest night of its month?
Because it is fixed to the new Moon, when the Moon's lit half faces entirely away from us.
On that night the Moon is in the daytime sky and offers no light at all after sunset, so the countryside is as dark as it ever gets. Lighting rows of lamps has an obvious effect that it would not have on a full-Moon night.
And because the festival follows the Moon rather than the Sun, its date on the Gregorian calendar moves each year — while Makar Sankranti, fixed by the Sun's position, stays put on almost the same date.
On that night the Moon is in the daytime sky and offers no light at all after sunset, so the countryside is as dark as it ever gets. Lighting rows of lamps has an obvious effect that it would not have on a full-Moon night.
And because the festival follows the Moon rather than the Sun, its date on the Gregorian calendar moves each year — while Makar Sankranti, fixed by the Sun's position, stays put on almost the same date.
Key takeaways
Calendars and timekeeping: quick revision
- A day is one rotation of the Earth, a month comes from the Moon's orbit (about 29 and a half days), and a year is one revolution around the Sun (about 365 and a quarter days).
- A solar day is measured between successive local noons, located by the shortest shadow of a gnomon.
- A lunar calendar keeps the Moon's phases but runs about 11 days short, so the seasons drift; a solar calendar keeps the seasons fixed; a luni-solar calendar does both by adding an extra month.
- Leap years exist because of the leftover quarter-day: 366 days once every four years, with the extra day in February.
- The Indian National Calendar is solar, based on the Saka era, starting with Chaitra near the spring equinox.
- Festivals follow either the Moon (Diwali on new Moon, Holi on full Moon) or the Sun (Makar Sankranti at the solstice turn), and satellites are launched for weather, communication, navigation and remote sensing.
You will remember all of this far better after answering five questions on it than after reading it twice.
- A solar day is measured between successive local noons, located by the shortest shadow of a gnomon.
- A lunar calendar keeps the Moon's phases but runs about 11 days short, so the seasons drift; a solar calendar keeps the seasons fixed; a luni-solar calendar does both by adding an extra month.
- Leap years exist because of the leftover quarter-day: 366 days once every four years, with the extra day in February.
- The Indian National Calendar is solar, based on the Saka era, starting with Chaitra near the spring equinox.
- Festivals follow either the Moon (Diwali on new Moon, Holi on full Moon) or the Sun (Makar Sankranti at the solstice turn), and satellites are launched for weather, communication, navigation and remote sensing.
You will remember all of this far better after answering five questions on it than after reading it twice.