Observations

# How long does the Earth take to go around the Sun and what causes the seasons?

An informal science survey at Bondi Beach conducted by COSMOS magazine asking people: How long it does it take for the Earth to go around the Sun?

A recent survey on science literacy conducted on behalf of the Federation of Australian Scientistic and Technological Societies (FASTS) and the Australian Academy of Science found that only 61% of Australians know that the Earth takes one year to travel around the Sun. This survey was followed up by COSMOS magazine with similar results as shown above.

The answers to a second question at Bondi Beach were even more disturbing with very few people being clear on what causes the seasons. This is in accord with the small surveys I had conducted over many years with WEA adult education classes at Sydney Observatory. Those groups knew how long the Earth took to travel around the Sun, but there was always a small group who were confused about the seasons as well as the phases of the Moon.

In this blog, we have covered the phases of the Moon in the previous post, so here we will briefly discuss the questions posed in the two videos above.

How long does the Earth take to travel around the Sun? Obviously, the answer is one year or 365.25 days. It is not so simple though as there are a number of definitions of a year. For example,

Tropical year, which is from equinox to equinox, that is from the time the Sun crosses the celestial equator from south to north to the next time 365.24219 days

Sidereal year, from one time a particular star is in a given position to the next time 365.25636 days

Anomalistic year, from the time the Earth is at its closest to the Sun to the next time 365.25964 days

What causes the seasons?

The Earth in southern summer and winter. Image Nick Lomb and Microsoft Clip art

The short answer is the tilt of the Earth or put more impressively, the obliquity of the ecliptic. The axis of the Earth is tilted by 23.4 degrees to the plane in which it travels around the Sun, the ecliptic. For most purposes we can assume that the Earth’s axis keeps pointing towards the same spot in space as it moves around the Sun. The tilt than means that in one position the tilt of the southern hemisphere is towards the Sun. That is summer in the southern hemisphere with the Sun appearing high in the sky during the day. Six months later, the tilt is now away from the Sun in the southern hemisphere and we have winter with the Sun low in the sky during the day.

The Earth does have an oval shaped path around the Sun so that one time during the year it is at its closest to the Sun and one time it is at its furthest. The difference in distance is only three per cent so that the shape of the path is NOT responsible for the seasons. It does, however, have a noticeable effect on the length of the seasons. When the Earth is closest to the Sun in early January each year it is moving at its fastest for the year. Hence our summers are shorter than our winters. In the northern hemisphere it is the other way around.

After reading this post, I am sure that all readers would be able to correctly and fully answer the questions posed in the two videos!

## 65 responses to “How long does the Earth take to go around the Sun and what causes the seasons?”

• What I find most fascinating is that the tilt of the earth’s axis, as it changes thus causing our 4 distinct seasons, happens to exactly correspond to our journey around the sun. Think about that
It takes roughly 365.25 days to go around the sun from point A to point A, and the earth’s tilt will be in the exact same spot as it was when it crossed point A one year earlier. That is most amazing to me.

but.. it’s because the tilt doesn’t change. only the position of the earth does.

• B Hunt says:

Have anyone ever considered that the Earth may be stationary and immoveable? (Yes as stated in the Bible) and if you search online there are sites that have done all the work for you and show the chapters that state this) According to NASA we spin at approx. 1038 mph at the equator all the time travelling through space at 67000 mph. The clouds and stars are stationary. I have never felt any movement. Water will always remain level. “Research Flat Earth”. We have been sold a lie.

• Andrew Jacob says:

B Hunt, Ha! And I worry that you have been sold a joke. The Earth is a sphere, it rotates and it revolves around the Sun. All the clouds and stars I see move. When I sit in a car or bus, I feel the bumps and the changes in direction but I cant feel the forward speed. Can you? Likewise, with no bumps or direction changes, I cant feel the movement of the Earth. But in 1727 James Bradley proved the Earth moves when he explained the aberration of starlight. Many millenia ago some thought the Earth to be stationary but now we have grown up, done the research and we know better.

• Uriel Parinas says:

The Earth is an oblate spheroid,
It’s not a perfect sphere.

• Andrew Jacob says:

Uriel, Thanks, you are right, and its a very lumpy oblate spheroid at that!

• Josh M says:

Andrew and Uriel, you are both wrong. To our tiny perspective our environment is quite lumpy. However, the perfection of our massive spinning spheroid rock ball is so smooth, it would appear the archetype of a geometric sphere from far away.

• FishQuak says:

I support B Hunt. It was also recently discovered by NASA that jupiter doesn´t orbit the Sun. Besides, if the Earth were to rotate and orbit the Sun, we would feel movement. And for thousands of years, humans have believed that the Sun orbits the Earth. Do you believe in evolution? Thatś a joke. People were stupid, AND STILL ARE. So why can´t we say the same about the theory of the Earth orbiting the Sun? Chimps to humans? I believe the definition of evolution to some is retarded. It actually means to change to adapt to the area, or evolve in to a more advanced field. It doesn´t mean to change from one species to another. The f—? ¨Done the research and we know better.¨? People believe that everything came from fish. Is that researching. Cause I know damn well it isn´t. That´s basically me saying whales came from fish, because they have fins. And because they live in water. And because they breathe.

• Andrew Jacob says:

FishQuack, If you had any integrity you would identify yourself honestly and not hide behind a (appropriate) pseudonym. Your ignorance is astounding. You might consider learning the difference between belief and evidence for a start. And then consider that it takes many years and intense effort to achieve high-quality research results – one sentence is not research! And I think the rest of humanity would appreciate it if you weren’t so rude about them.

• Nissim Hadar says:

How do you fly from New Zealand, or South Africa, to Chile?

• Pinky-Winky says:

Green cheese anyone? Nice observations A.J., with easy, reproducible and tested explanations. You don’t feel like you’re moving at 500 mph sitting your comfy seat on an airplane either – until you hit a mountain! Talk to me after you’ve launched A.J. or at least have read a ninth grade physics book, maybe on your next flight.

• Andrew Jacob says:

Pinky-Winky, Well that’s the point isn’t it. Whether you are on a bus, airplane or Earth you don’t ‘feel’ the motion, yet you are moving.

• Aryan says:

What will be the duration of a day if the distance between earth and sun is reduced to half?

• Andrew Jacob says:

Aryan, the Earth’s rotation on its axis (its day) is independent of its revolution about the Sun (its year). So moving Earth closer to the Sun would have no affect on the length of its day. At least no affect you would immediately notice. The higher tidal forces from the Sun, however, would increase the rate at which earth’s spin is slowing down. This is already happening due to the Moons tidal forces. The effect on the rest of the solar system, in particular Venus, would be significant!

• monu reshawat says:

i think that the earth 365 days 5 hours 48 minutes and 56 second to complete one revolution

• How much time does the earth take to complete one rotation?(with seconds)

• Andrew Jacob says:

Kavi, Earth rotates once in 24 hours with respect to the Sun but in 23 hours and 56 minutes with respect to any particular star. I’ll leave it as an exercise for you to work out how many seconds that is.

• anurag says:

I think it will take 365 days 5hours 49mins and 12secs as they are 97 leap years in 400 years so time taken will be 365days and 97/400 days i.e. 5hrs 49 mins 12 secs.

yes, there are only 97 leap in 400 years not 100 as from 2000-2400 AD. 2100,2200 and 2300 will not be leap years because when there is two zeroes behind one year it is divided by 400 not by 4 so because of that the answer will be the above one

• Armel smith says:

My teacher at school said that while earth is rovealing around the sun that what cause the seascones

• Andrew Jacob says:

Armel, As the post explains it is the tilt of the Earth (23.4-degrees) in combination with the revolution of Earth around the Sun that causes the seasons.

• Sam says:

If the earth is turning on its axis as fast as being purported, shouldn’t we be seeing flashes of darkness and light, instead of hours of night and hours of day?

• Andrew Jacob says:

Sam, this post is about the length of the year and the seasons. However, given that a day is 24 hours long then we get about 12 hours of daylight and about 12 hours of nighttime each day – give or take a bit for the changes due to the seasons and your latitude.

• Nonso Roy says:

I learnt that the earth is about 93,000,000 miles away from the sun, that she revolves around the sun once a year at a speed of 66,600 miles per hour to maintain that distance and that it tilts in relation to the sun at an angle of 23.5 degrees. Now I am more particular about the figures. Are they constant year in year out? What would be the result of a deviation from such figures? Thank you.

• Andrew Jacob says:

Nonso Roy, Yes, there are changes in these numbers which occur over different periods of time. It gets complex and could take a lifetime to study in detail! One place to start is with the Milankovic cycles, specifically with the changes mentioned in Earth’s orbit and tilt. But note that while measuring the changes is possible, determining the effects of those changes is more difficult.

• Unnikrishnan says:

It is clear that equinox occurrs on March 21 and September 23rd every year. Let us use this data to make calendar using astronomy.

Other wise no basis for calendar.

• Antoinette el Ichati says:

If Earth is tilted at 23.5 degrees with the tilt always in the same direction as it goes around the sun, doesn’t that mean that the same hemisphere is always the one furthest from the sun and the other hemisphere is always the closest? This has got nothing to do with the variation of distance the Earth as a whole is away from the sun due to its oblique path.
So how are seasons happening?

• Andrew Jacob says:

Antionette, Earth’s axis is tilted in the same direction with respect to the distant stars, not with respect to the Sun. The diagram in this post about the seasons shows the situation.

• ARVIND says:

Respected sir..
Does earth take 24 hrs to complete one year or one revolution ( from solar view)

• Andrew Jacob says:

Arvind, From the point of view of the Sun the Earth takes 24 hours to complete one rotation.

• Paul says:

Hi, does the earth really rotate? How come we dont notice the spinning either around itself or around the sun?(as well as we always see one side of the moon and sometimes during the day we see thw moon)
And why we always use words as the sun sets and the sun rises as we know the earth turns around the sun and the sun doesnt move?
Tx

• Andrew Jacob says:

Paul, Yes, the Earth really rotates. Its just so big and you are on it, and moving along with its surface, that you don’t ‘feel’ the motion. Sometimes in a car on a smooth highway you can close your eyes and not ‘feel’ that you are moving! It’s the same with the Earth. You can of course see the Earth turning when you watch the stars & Moon rise or set. And we see different constellations overhead during the course of the year showing we are really moving around the Sun. I guess we use the terms ‘set’ and ‘rise’ for historical reasons. The proper term for sunset should be a “terrestrial occultation of the Sun” and sunrise is the end of the occultation or , perhaps, “the revelation of the Sun”! Maybe its easier to stick with sunrise and sunset!

• bob says:

the earth really does rotate bob the builder

• Dev says:

Can the earth be identified being in/moving over the fixed zodiac each month,just like planets are identified being in/moving over the fixed zodiac?

• Andrew Jacob says:

Dev, From the point of view of an observer on the Earth the planets, Moon and Sun move in front of the distant stars. The stars they pass form a band across the sky, called the zodiacal band, not far from the path the Sun takes across the sky. You can place the Earth in this band at a point 180-degrees from the Sun’s position in the band. In astronomy we call this the antisolar point. If you were on the Sun looking towards Earth, the Earth would appear in the zodiacal band at the antisolar point.

• santosh says:

In which month eirth completes a round. around the sun

• Andrew Jacob says:

Santosh, I’m not sure if I am answering your question but there is no real beginning or end to Earth’s orbit around the Sun, it is like a ring!

• Zytaris says:

I have found it interesting to note, that the beginning of the year is January, and it is among the time area which the perihelion occurs, when the earth and the sun are closest together. This area of time may be why chosen to be the beginning of the orbit?… as a sort of reference point, knowing that the orbit is not a strict circle path.

• Andrew Jacob says:

Zytaris, interesting idea but I think it was more of a historical decision, occurring long before anyone knew the true nature of Earth’s orbit, or even that Earth orbited the Sun.

• Olivia says:

Well, according to our calendar the Earth completes a round in december but like Andrew Jacob said, it’s just like a ring.

• Hi, hopefully you can answer this nagging I have. I find time quite spurious in and of itself and understand our version of time to have been constructed by us, by mainly studying the sun and the light and dark periods it provides us. we then developed numerous calendars from the rather intricate mayan calendar to the western version introduced by caesar and modified by gregory. what is nagging me is in your article you state that ”For most purposes we can assume that the Earth keeps pointing towards the same spot in space as it moves around the Sun.” so lets say month zero at 12pm we are in the height of day and the space we are facing is occupied by the sun. In an oval or elipse looking top down on our system this logic infers that 6 months later the same part of the globe would be at the same time i.e. 12pm midday facing away from the sun in complete darkness… which brings me to the second part of my conundrun which is I know we adjust clocks +or- 1 hour four ‘daylight saving’ why do we not infact add +/-12 hours or why does this not need continually adjusting during our trip around the sun? thanks in advance of your conclusions.

• Andrew Jacob says:

Sensi, Maybe our wording was confusing you. I have edited the sentence to say “For most purposes we can assume that the Earth‘s axis keeps pointing towards the same spot in space as it moves around the Sun.” We then define midday to be when the Sun is at its highest point in the sky, or equivalently when it is to the north of your location. We do not need to add +/-12 hours because our clocks (which run on solar time) are already accounting for Earth’s change in position as we orbit the Sun. There is an alternative time called sidereal time, which is measured against the stars, for which we do need to add 4 minutes every day to reach solar time.

• subash says:

If the earth is round like a globe, how is it that we do not fall off?

• Andrew Jacob says:

Subash, that question is off-topic for this page. However, all material objects have a centre of mass, and all objects are attracted to each others centres of mass trough the force of gravity. The centre of mass of the Earth is its centre, far below your feet). Therefore wherever you are on Earth you are being pulled toward the Earth’s centre, ad not in any other direction. The only way to ‘fall off’ the Earth is to give yourself such a big push that you don’t come back – that is what a rocket does to an astronaut.

• Steve says:

Picture a cannon. It fires a cannonball. Not much power, so it falls straight to the ground. That’s gravity. It’s why your feet are planted to the ground. Now imagine a much more powerful cannon. It shoots a cannonball for miles. But it’s curved trajectory will still see it fall to the ground. That is gravity. Eventually we developed spacecraft that still follow that trajectory but their arc is the same as the Earth’s curvature so it keeps falling back to Earth, but until it slows down it stays in orbit.

• Olivia says:

…Gravity which basically keeps us within the atmosphere pulling us towards the ground.

• Gordon Tippett says:

What happens to the 45 minutes and 48 seconds beacuse leap year only accounts for 6 hours extra?

• Andrew Jacob says:

Gordon, As the post states the Tropical year is 365.24219 days long, or slightly shorter than 365 days and 6 hours (or 365.25 days). Adding a leap year every fourth year would overcompensate for the difference and add about 3 days in every four hundred years. So the ‘leap year’ rule states “Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is.” [Introduction to Calendars, USNO]. With this additional correction in place (giving an average year of 365.2425 days) the overcompensation is only 1 day in several thousand years. This difference is too small to worry about just now!

• Shahid Parvez Chaudhari says:

Your calculation is perfect as per mathematics.

• Michael says:

I was under the impression that the entire universe is moving, if so wouldn’t we see a change in the alignment of stars and other galaxies at some point or even a few degrees?

• Andrew Jacob says:

Michael, Yes everything in the universe is moving in some way. Usually one object is in orbit about another. However, the distances to the stars are so great that the movements are not apparent to your eye. It requires telescopes and quite refined measurements to detect the motions during a single year, although it is routinely accomplished. There are some stars whose motion you could observe by eye through a telescope over the course of a few years. And some naked eye stars could be seen to change position if you could watch them for a few hundred years!

• Warren says:

Question:
Why does the cycle of seasons, the cycle of the earth’s tilt on its axis, correspond exactly with one rotation of the earth around the sun? Is this phenomena something seen throughout our solar system with other planets?

• Andrew Jacob says:

Warren, this is because Earth’s tilt is fixed relative to the distant stars, i.e. Earth’s axis points towards the same point in space as Earth revolves around the Sun. Yes, other planets have their axes tilted, some more or less so than Earth. For example Mars is tilted at about 25-degrees.

• 365 days, apparently 1 day to cross 1 angle

• I am just leaving Norway where I was told previous night sunset was midnight. I know earth goes around the sun and that takes one year. I didn’t stay awake til midnight then but know from a 2a.m. Wake up it should be safe to assume that the sun didn’t actually set at all. I can’t find an illustration of the northern most axis point of the earth being subjected to this activity. Can you expand?

• Andrew Jacob says:

Hello Alison, it is true that during the northern summer the Sun can remain above the horizon, neither setting nor rising. This only happens if you are further north than the Arctic circle. The number of days, or perhaps I should say the number of 24-hour periods, it remains above the horizon during the summer depends on your latitude. The further north you are the longer it remains above the horizon without setting or rising. The same effect occurs in the southern hemisphere during the southern summer, but there are far fewer people in the region (solely the Antarctic continent) who experience this.

The reason you would have seen the Sun up at midnight is that the 23.4 degree tilt allows you to, just, peek over the north pole towards the Sun. I can only find an illustration for the southern hemisphere. In this figure I think you can see how sunlight ‘reaches’ past the south pole for someone very close to that pole.

Explaining and understanding these kinds of astronomical phenomena can be difficult in words and with 2-dimensional images. I find using balls and bright desk lamps, or torches, in a darkened room is very helpful in comprehending the geometry!

• Nandini Sharma says:

It takes 365 days , 6 hours , 45 minutes and 48 seconds for Earth to revolve around the Sun . Revolution causes seasons .

• Andrew Jacob says:

> Well, not quite. The revolution by itself would not cause the seasons. The 23.4 degree tilt of Earth’s axis is the essential ingredient.

• Meraj Kashmiri says:

Hi
Nandni Ji, I hardly agree with your figures. u say sun take
365 days OK
06 hours OK which causes a leap year.
then u say 45 Mins and 48 secs. that is 2748 secs. I DON’T AGREE.
Becouse in a day there are 24*60*60=86400 Secs.
therefore 86400/2748=31.44. Means after every 32 years one more day (i, e, 367th day) should be added to the year. That February should be 30 days after every 32 years.
Thanks

• Roberto says:

A very good article. I agree that the cause of the seasons is not well understood among the general public.

• Gareth says:

Thanks for an informative article. Is it possible for a planet to have vastly different tropical, sidereal, and anomalistic years?

• Hello Gareth. Great question. The difference between tropical and sidereal years arises from the Earth’s slow precession (wobble of the axis) with a period of around 26,000 years. If there was a planet somewhere around another star with a much shorter period of precession then theoretically it would be possible to have a much greater difference between the periods.

With regard to anomalistic years they differ from tropical years because of a rotation in the orientation of the planet’s orbit. Again, it would be possible for a hypothetical and pathological planet to have a quicker rotation of the orbit and hence a greater difference between the two kinds of year.

• Swissgecko says:

Hang on – isn’t the path of the Earth around the sun an ellipse, and not an oval?

• Hello Swissgecko. You are quite right the orbit is an ellipse, but when writing for this blog I try to avoid technical tems like ellipse or orbit. In any case, an oval with two axes of symmetry is very close to an ellipse.

• Mr. ? says:

> What happen to the teachings that it took the earth 4 years to orbit the sun 1 time? What happened to the earths wobble back and forth on its axis that caused summer and winter? Why would I even think that if it hadn’t been from a teacher in school who taught that? Interesting ..indeed.

• Andrew Jacob says:

Mr ?, I’m afraid what you say your teacher told you was incorrect. But I’m pleased we were able to correct those misconceptions.