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

September 15, 2010

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!


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

  • 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.

    • 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, 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.

    • 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!

  • 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?

    • 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!

  • 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?

    • 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.

  • 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?

    • 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!

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

    • > 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.

    • 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 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.

    • 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.

      • > 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.

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

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