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

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

Published by Nick Lomb on September 15, 2010 18 Comments

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 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!

Leave a Reply

18 Responses to “How long does the Earth take to go around the Sun and what causes the seasons?”

  1. September 15, 2015 at 9:19 am, Warren said:

    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?


    • September 16, 2015 at 4:39 pm, Andrew Jacob said:

      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.


  2. May 29, 2015 at 2:23 pm, redgiant said:

    365 days, apparently 1 day to cross 1 angle


  3. May 22, 2015 at 8:45 am, sylvia said:

    this is the best website ever


  4. May 22, 2015 at 7:24 am, lorra said:

    this is not helpful


  5. May 20, 2015 at 8:32 pm, Alison Sutherland said:

    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?


    • June 02, 2015 at 5:20 pm, Andrew Jacob said:

      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!


  6. June 02, 2014 at 1:56 am, Nandini Sharma said:

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


    • June 02, 2015 at 4:41 pm, Andrew Jacob said:

      > 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.


  7. February 21, 2013 at 3:28 pm, kyle skerke said:

    Like the video <3


  8. April 20, 2012 at 1:27 pm, Yrufjewk said:

    terrible info


  9. September 17, 2010 at 9:00 am, Roberto said:

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


  10. September 16, 2010 at 2:33 pm, Gareth said:

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


    • September 16, 2010 at 3:47 pm, Nick Lomb said:

      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.


  11. September 16, 2010 at 1:47 pm, Swissgecko said:

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


    • September 16, 2010 at 2:05 pm, Nick Lomb said:

      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.


      • October 21, 2015 at 4:36 am, Mr. ? said:

        > 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.


        • October 21, 2015 at 12:08 pm, Andrew Jacob said:

          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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