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Our orbit and the missing days of winter

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Stephen Marsh Stephen Marsh | 17:30 UK time, Wednesday, 6 April 2011

d ~ 246'988'800 km: day 96

As we say goodbye to winter and enjoy the start of spring there's one thing here in the Northern Hemisphere that most of us don't notice, but never the less we should be thankful for. Our winter ends sooner than it does in the southern hemisphere - 4 days shorter in fact. Which is good news for us because we get four less days of colder weather but bad news for people down-under who get four days more.

It's all down to our orbit around the sun. We can't really see it, but it's not a perfect circle, it's an ellipse with the sun slightly offset in the middle. This was first explained by Johannes Kepler in 1609 in his first law of planetary motion.

Johannes Kepler 1st law of planetary motion

The part of the orbit when the earth is closest to the sun is called Perihelion and currently it occurs during the Northern Hemisphere winter. At Perihelion the earth is 5 million kilometres closer to the sun than in July when it's at it's furthest.

Johannes Kepler 2nd law of planetary motion

So why does that have an effect on the length of the winter. Well that's down to Keplers second law. This states that a line joining a planet and the Sun, sweeps out equal areas during equal intervals of time. Ok that's the scientific description of the law but what does that mean when it comes to our journey around the Sun? Put simply the law explains that when the planet is closer to the sun it moves faster round its orbit, and when it is further away it moves slower.

So during Perihelion and the northern hemisphere winter the planet is moving faster making the season shorter. The southern hemisphere winter is four days longer because it occurs at aphelion when the planet is further away from the sun and therefore moving slower.

But there's a flip side to this because that means that the Northern Hemisphere summer is four days longer than the southern hemisphere summer. Which helps explain why July is our planet's warmest month, At that point the Northern continents that are pointing towards the sun get a few days more to bake in the sun and raise the average temperature of the entire globe.

Most of the other planets in our solar system have orbits that are more elliptical than Earth's. The dwarf planet Pluto's orbit is the most eccentric of all and is so lopsided that at its Perihelion it is actually closer to the sun than Neptune.

Mars's orbit is not as elliptical as Pluto but a lot more than Earth's. At Perihelion Mars is 43.5 million kilometres closer to the sun than at Aphelion and receives 40 percent more sunlight, with an abundant rise in temperatures of around15-20 degrees. The larger degree of eccentricity in the orbit also affects the lengths of the seasons because the planet moves slower at Aphelion, as it is further away from the sun. As a result the Northern Hemisphere summer is not four days longer than the northern winter, but 25 days longer. This has a big impact on the Red Planet's seasons. Northern summers are long and cool while the winters are short and mild. Conversely for the southern hemisphere, summer is short and relatively hot while winter is long and cold.

So we should be quite glad that our orbit is only slightly elliptical - we barely notice our shorter winter, but if we were on Mars we certainly would.


  • Comment number 1.

    [Unsuitable/Broken URL removed by Moderator]That is fascinating. I had no idea that the Northern Hemisphere had a longer summer than the Southern Hemisphere.

    This gives me the idea to write a post on one of my Travel Blogs and poll my readers. With a large network of RTW travellers, I would be curious to know just how many of them are aware of this. I have a suspicion that many of them do and I will end up looking naive. Either way it is a fun fact to share. I will link back to this article to prove I am not mad.

    Mike Collins
    editor of
    [Unsuitable/Broken URL removed by Moderator]


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