Motions of the Earth: Rotation, Revolution and the Seasons

The Earth is never still. It does two things at the same time.

It spins on its own axis, and it travels around the Sun. Each motion has its own job.

1.1 What is rotation?

Rotation is the spin of the Earth on its own axis. The axis is an imaginary line through the North and South Poles.

One full spin takes about 24 hours. The Class 6 geography book calls this period the earth day. Rotation gives us day and night.

1.2 What is revolution?

Revolution is the journey of the Earth around the Sun. The path it follows is called its orbit.

One trip takes 365 and a quarter days. That is one year. Revolution, together with the tilt of the axis, gives us the seasons.

1.3 Which motion causes what?

Many exam errors come from mixing up the two. Keep this short list in mind.

• Rotation gives day and night, and it sets our clocks.
• Revolution gives the year and the leap year.
• Revolution plus the tilt gives the seasons.
• The tilt alone does not change through the year. Its direction stays fixed.

The Sun lights only one half of the Earth at a time. As the Earth spins, each place turns into the light and then out of it.

2.1 What is the circle of illumination?

At any moment, half of the globe faces the Sun. That half has day. The other half is in the dark and has night.

The line that divides day from night on the globe is called the circle of illumination. The book notes that this circle does not match the axis.

A place that turns into the light sees sunrise. A place that turns out of it sees sunset.

2.2 Is a day really 24 hours?

There are two ways to count a day. They differ by about four minutes.

• A solar day is one cycle from daytime to the next daytime. It is about 24 hours.
• A sidereal day is one full spin measured against the stars. It is almost exactly 23 hours and 56 minutes.

NASA’s fact page rounds the spin to 23.9 hours.

For exams, write 24 hours as the length of a day. Use 23 hours 56 minutes only when the question says “relative to the stars”.

2.3 How fast does the Earth spin?

The speed depends on where you stand. At the equator, the ground moves west to east at about 1,675 km per hour (NASA GPM). National Geographic says about 1,600.

You do not feel this motion. Everything around you, including the air, moves with you.

2.4 Who explained that the Earth rotates?

The Class 6 book has a “Do you know” note on this. It says the ancient Indian astronomer Aryabhata stated that the Earth is round and rotates on its own axis.

So the daily rising and setting of the Sun comes from our own spin. The Sun does not travel around us each day.

Do not write “first” in an answer unless the question does. The book only says that he stated it.

2.5 What else does rotation do?

Rotation bends moving air and water. In the Northern Hemisphere, they turn to the right. In the Southern Hemisphere, they turn to the left.

This bending is called the Coriolis effect. It shapes winds and ocean currents. You can read more about winds in the lesson on climatology.

The Earth turns 360 degrees in about 24 hours. That fact is the base of all time zones.

3.1 Why is one hour equal to 15 degrees?

Divide 360 degrees by 24 hours. You get 15 degrees every hour. That is also 1 degree in four minutes.

So places that differ by 15 degrees of longitude differ by one hour in local time.

You can see how longitude lines work in the lesson on latitudes, longitudes and heat zones.

3.2 What is Indian Standard Time?

India is wide, so it uses one time for the whole country. It takes that time from the longitude 82 degrees 30 minutes East. This is the standard meridian of India.

That meridian lies east of Greenwich. So India is 5 hours 30 minutes ahead of Greenwich Mean Time. The maths agrees: 82.5 divided by 15 is 5.5 hours.

Today IST is set at 5 hours 30 minutes ahead of UTC. A government laboratory, CSIR-NPL, keeps it.

3.3 What is the International Date Line?

The date must change somewhere. By agreement, it changes near 180 degrees longitude.

NOAA says the line was established in 1884. It runs through the mid-Pacific. If you cross it going west, it becomes one day later.

It is not a straight line. It zigzags around some borders, such as eastern Russia and the Aleutian Islands. It also has no legal international status. Countries choose which date they keep.

The Earth goes round the Sun along a fixed path. This journey sets the length of our year.

4.1 What is the Earth’s orbit like?

The orbit is not a perfect circle. It is an ellipse, which is a slightly stretched circle. The Class 6 book shows this in a figure.

The Earth moves along it at about 29.78 km per second on average. You do not feel this either.

4.2 Why does a year have 365 and a quarter days?

One trip around the Sun takes about 365.25 days. Our calendar year has 365 days. We leave out the extra quarter day.

The book says we ignore six hours each year for convenience. NASA’s JPL gives the orbit time as 365.2422 days, which it calls about six hours longer than 365 days.

Those hours do not vanish. They pile up. After four years they make about one extra day.

4.3 Which years are leap years?

Every fourth year, February has 29 days instead of 28. That year has 366 days and is called a leap year.

The full rule has a twist for century years.

• A year divisible by 4 is a leap year.
• A century year, such as 1900, is not a leap year.
• A century year divisible by 400 is a leap year again.

So 1900 was not a leap year, but 2000 was. Also 2100, 2200 and 2300 will not be.

The twist exists because adding a day every four years gives a little too much. It adds about three extra days every 400 years.

The axis is not straight up and down against the orbit. It leans. This lean is the key to the seasons.

5.1 How big is the tilt?

NOAA’s NESDIS says the axis is tilted about 23.45 degrees from the perpendicular to the orbital plane. NASA says 23.4 degrees with respect to the plane of Earth’s orbit.

The orbital plane is the flat surface on which the Earth goes round the Sun. Think of it as a giant tabletop.

The tilt is why the Tropic of Cancer lies at 23.5 degrees North and the Tropic of Capricorn at 23.5 degrees South.

5.2 Why does the school book say 66.5 degrees?

The Class 6 book says the axis makes an angle of 66 and a half degrees with the orbital plane. This is the same tilt, measured from a different side.

Look at the tabletop. The axis leans 23.4 degrees from the perpendicular. The angle between the axis and the tabletop is what remains of 90 degrees. That is about 66.5 degrees.

So both numbers are correct. One is measured from the perpendicular. The other is measured from the plane.

The same figure, 66.5 degrees, is also the latitude of the Arctic Circle.

5.3 Does the tilt change as the Earth moves?

No. The tilt points the same way all year. The book says the Earth is inclined in the same direction throughout its orbit.

Think of a spinning top that keeps leaning toward one corner of the room. You walk around it. Sometimes the lean points toward you. Sometimes it points away.

That is exactly what happens with the Sun.

The seasons come from the tilt and the revolution working together. Neither one is enough alone.

6.1 How do tilt and revolution make the seasons?

The tilt keeps pointing the same way. The Earth moves round the Sun. So the two hemispheres take turns leaning toward the Sun.

• When the Northern Hemisphere leans toward the Sun, its days are longer. The Sun’s rays are more direct. It is summer there.
• Six months later, the same hemisphere leans away. Days are shorter and rays are slanting. It is winter there.
• The Southern Hemisphere always has the opposite season.

6.2 Is it hotter in summer because the Earth is closer to the Sun?

No. This is a very common wrong idea. The Earth does have a nearest point and a farthest point on its orbit.

• The nearest point is called perihelion. It is about 147.095 million km from the Sun and falls in January.
• The farthest point is called aphelion. It is about 152.100 million km from the Sun and falls in July.

Look at the months. The Earth is closest to the Sun in the Northern winter. It is farthest in the Northern summer.

NOAA puts it in one line: the Earth’s distance from the Sun is not the cause of the seasons.

A solstice is a day when the Sun’s direct rays reach their farthest point from the equator. It happens twice a year.

7.1 What happens at the June solstice?

On about 21 June, the Northern Hemisphere is tilted toward the Sun. The Sun’s rays fall straight down on the Tropic of Cancer. Straight down means a 90-degree angle.

This is the Summer Solstice for the north. It brings the longest day and shortest night there. In the Southern Hemisphere, it is the Winter Solstice, with the longest nights.

National Geographic says the June solstice falls between 20 and 22 June. The exam date is 21 June.

7.2 What happens at the December solstice?

On about 20 to 23 December (National Geographic), the South Pole tilts toward the Sun. The Sun’s rays fall straight down on the Tropic of Capricorn, at 23.5 degrees South.

The Southern Hemisphere now has summer with its longest day. The Northern Hemisphere has its Winter Solstice and its shortest day.

7.3 Do the poles really get six months of daylight?

The Class 6 book says places beyond the Arctic Circle get continuous daylight for about six months. This is a simplification.

National Geographic describes the midnight sun as the Sun staying visible at midnight in the weeks around the summer solstice. The opposite is the polar night. In exams, use “continuous daylight” and avoid exact day counts.

An equinox is a day when the Sun’s direct rays fall on the equator.

8.1 When do the equinoxes happen?

Around 21 March and 23 September, the Sun is straight over the equator. Neither hemisphere leans toward the Sun.

Day and night are then nearly equal everywhere. They are not exactly equal, because the air bends sunlight (atmospheric refraction).

National Geographic puts the equinoxes around 21 March and 22 September. The exam dates are 21 March and 23 September.

8.2 Which place gets the lowest Sun angle at the June solstice?

Start with one fixed fact. At the June solstice, the noon Sun is straight overhead at 23.5 degrees North. That is the Tropic of Cancer.

For any other place, count how many degrees of latitude it lies from 23.5 degrees North. Then subtract that number from 90. The result is the angle of the noon Sun above the horizon.

• Tropic of Cancer: 0 degrees away, so the angle is 90 degrees.
• Equator: 23.5 degrees away, so the angle is about 66.5 degrees.
• Arctic Circle: 43 degrees away, so the angle is about 47 degrees.
• Tropic of Capricorn: 47 degrees away, so the angle is about 43 degrees.

Among these four, the Tropic of Capricorn has the lowest Sun angle. The Sun angle falls as the distance from the overhead point grows.

This matters for weather and climate too. You can read how in the lesson on astronomy fundamentals and in the lesson on the equator.

Most mistakes come from mixing up numbers and causes. Check each point below before you answer.

9.1 Which wrong statements should you spot?

• The tilt: it is 23.4 degrees from the perpendicular, and 66.5 degrees with the orbital plane. It is never 90 degrees with the plane.
• The day: 24 hours for a solar day, and 23 hours 56 minutes for one spin against the stars.
• The year: 365 and a quarter days. Leap years follow the century rule.
• The seasons: the tilt and revolution cause them. Distance from the Sun does not.
• The dates: solstices on 21 June and 22 December, and equinoxes on 21 March and 23 September.
• The Sun angle: the lowest noon angle at the June solstice is at the Tropic of Capricorn among the four common choices.
• The people and places: Aryabhata stated the rotation. IST is based on 82 degrees 30 minutes East.

For more practice, use the random questions link at the bottom of this page.