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 7 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-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

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

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

    Like the video <3

    Reply

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

    terrible info

    Reply

  3. 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.

    Reply

  4. 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?

    Reply

    • 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.

      Reply

  5. 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?

    Reply

    • 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.

      Reply

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