Perihelion: The Earth is on its way back out again

By Phil Plait | January 4, 2012 7:02 am

Tonight at roughly 01:00 GMT (08:00 p.m. Eastern time), the Earth will be at a special place in its orbit: perihelion, the closest point to the Sun. Our orbit around the Sun is not a circle, but actually an ellipse, and in early January every year the Earth’s motion sweeps us closest to our favorite star. We’re only a couple of million kilometers closer than average so it’s a small difference, and not one you’d notice unless you were paying very close attention.

If you want a little more precision, the distance from the center of the Earth to the center of the Sun will be 147,097,206.9 km at that moment. More or less.

Apropos of this, I wrote a guest post about perihelion and what it means for the wonderful BBC blog called 23 Degrees. This is the companion blog for a TV documentary series they’re making (to air later this year) where they traveled the globe to film meteorological and astronomical events that occur during the course of one year. And since they began this journey at perihelion last year, I’m honored to have this anchor position.

So to speak, of course. Anyway, check the Related Posts links below for lots more about past perihelia (they’re listed in reverse chronological order). It’s always fun to write about it, and always fun to learn more about this spinning ball of rock we live on and the giant ball of plasma it orbits.


Related posts:

At the bottom of Earth’s orbit
Perihelion!
Does this perihelion make my Sun look fat? (with cool math!)
Don’t you just hate perihelion?
Approaching the Sun

CATEGORIZED UNDER: Astronomy, Cool stuff
MORE ABOUT: 23 Degrees, BBC, perihelion

Comments (32)

  1. Ella

    This might be a bit of a noob question, but when the Earth approaches perihelion would that mean that places like Australia, which experience summer during that time, would have a slightly more hotter summer compared to the Northern Hemisphere’s summer because the sun is closer?

  2. This is also the 1st time on the modern calendar that Earth’s perihelion falls on Jan 5th as reckoned in UTC… will do so again in 2020 and with greater frequency thereafter.

    http://astroguyz.com/2012/01/02/astroevents-hunting-things-that-flash-in-the-january-sky/

  3. Puck

    Actually, Ella, nothing can be more hotter because that’s just bad grammar.

  4. Bill

    Perihelion Day! Always a good science teaching day for friends and family. Ask them, “Why are there seasons?” The most common answer is that we are closer to the sun in the summer and further away in the winter, always a teaching moment. You can also console them if they did not get the answer right: >90 % of Harvard graduates could not get the answer correct, and it was very difficult for them to learn the correct answer. I learned this in an in-service teaching class a few years ago. Bottom line: it’s really hard to unlearn something so get it right the first time.

    Also, beware of smugness: it always turns people off from learning science.

  5. I am always amused by facebook posts that talk about the earth being at a “perfect” distance from the sun, and if it varied by more than (some small number) life would be impossible…

    @ Ella (#1), I can’t find the reference, but I recall reading that there is indeed an effect from having a bit more radiant flux landing on Australia, but it’s not very significant (+0.3C to +3.0C [the number three seems to stick in my memory, I just can’t recall where the decimal point goes…]). Of course, this is complicated by the fact that there is more water surface in the southern hemisphere than in the northern, which also plays into that.

  6. hhEb09'1

    Interesting factoid: the earth’s orbit is an ellipse, but it deviates from a circle by less than the diameter of the earth

  7. Tim Gaede

    Ella,

    Since perihelion coincides closely with the southern summer solstice (and aphelion with the southern winter solstice), one would reasonably expect that the southern hemisphere should experience more extreme seasons. But the southern hemisphere is dominated by oceans, which tends to moderate temperatures and as a result experiences less extreme seasons.

    I’ve read a nice article online that quantifies the seasonal variation for each hemisphere, but I cannot find it.

  8. Bud

    Anytime I hear an argument against government-supported broadcasting, I am reminded of the incredible work of the BBC in science reporting. Fantastic.

  9. ChazInMT

    @ hhEb09’1 Interesting Factoid 4U….The Earth is NOT 3,110,000 miles in Diameter, your Factoid is a Fictoid made up by you I’d guess.

    If you draw a circle on a piece of paper with a 1/4 inch (7mm) line the Earth does stay inside that circle.

  10. Scott R

    This is perhaps a naive question. Is there any significance to perihelion occurring near the solstices? Is this expected? Or is it coincidence?

  11. hhEb09'1

    I didn’t say it was! :)

    The semi-major axis of the earth’s orbit is 149598000 km, and it’s semi-minor axis is 149577000 km, so splitting the difference between them means a little over 10,000 km, much less than the diameter of the earth.

    You could draw a circle with a .0005 inch line and it would fit inside.

  12. Tim Gaede

    The deviation of Earth’s orbit from a circle is far less than its deviation of its distance to the sun.

    The “width” (double the semi-major axis) of the ellipse should be only 0.014% more than the “height” of the ellipse. The Earth is 3.4% farther from the Sun at aphelion than at perihelion.

    The difference between the semi-major and semi-minor axis is actually 21,000 km, which is more than the diameter of the Earth but still small compared to the distance to the Sun.

    I wonder if Kepler actually considered off-center circles before looking into ellipses.

  13. Pete Jackson

    Yes, the changing distance from the Earth to the Sun is caused much more by the fact that the sun is at a “focus” of the ellipse, rather than that the ellipse is slightly more squashed than a circle. The distance of a focus of an ellipse from the center of the ellipse is equal to the semi-major axis times the eccentricity, and for the Earth-Sun that is 149,598,000 km times 0.017 or 2,543,000 km.

    Kepler determined that planetary orbits were elliptical based on Tycho Brahe’s very precise observations of Mars, whose orbit is much more elliptical (eccentricity = 0.093) than the Earth’s orbit.

    The Earth is 3.4% (2*0.017) further from the Sun in July than in January and, by the inverse square law, the heat received from the Sun 6.8% greater in January than in July.
    By the fourth-power relationship between radiation energy and temperature, the Earth, if it were airless like the Moon, would have a mean temperature 1.7% greater, on the Kelvin scale, in January than in July. With the mean temperature being around 280K, this would
    be 4.8 degrees Kelvin warmer in January than in July, which is equal to 4.8 degrees Celsius or 8.6 degrees Fahrenheit.

    As has been pointed out, the oceans moderate this difference, especially in the southern hemisphere.

    But if you look at climate of towns and cities in Africa, near the equator, you can usually tell that July is a few degrees cooler than January. It would be a good science project to plot the difference versus latitude for a number of such towns and cities, and fit the trend line to see what the mean difference is right at the equator.

  14. Zippy the Pinhead

    Off topic, last week walking home from the supermarket I saw the new crescent Moon in conjunction with Venus, then 2 nights ago the almost half Moon in conjunction with Jupiter. And last night walking home from work I again saw Venus in the west, the Moon and Jupiter above, and also Sirius rising in the east.

  15. hhEb09'1

    @Tim Gaede: the difference is 21000 km, but you can draw the circle halfway between them, so that the deviation from the circle is less than 11000 km, smaller than the diameter of the earth.

  16. Bill Nettles

    “Interesting factoid: the earth’s orbit is an ellipse, but it deviates from a circle by less than the diameter of the earth”

    This is a prime example of a poorly stated idea. There are too many unstated assumptions, making it ill-defined. These types of statements lead to BAD ASTRONOMY.

    Misleading at best, here are two reasons I consider it wrong.
    1) You shouldn’t “split the difference” between the semi-major (a) and semi-minor(b) axes because they are already “split.” If you match a of the ellipse with r of the circle then when you get to b the difference is (like Tim Gaede says) about 21000 km.
    2) While the path defined by Earth’s orbit, if centered on a circle with same radius as the semi-major axis would have the 21000 km difference, the ORBIT of the Earth is NOT around the mid-point of the major axis. The orbit is around the Sun at the focus. Matching the Sun of Earth’s orbit with the Sun of a circular orbit, there are millions of km of difference.

  17. Bill Nettles

    “@Tim Gaede: the difference is 21000 km, but you can draw the circle halfway between them, so that the deviation from the circle is less than 11000 km, smaller than the diameter of the earth.”

    That’s a poor definition of your circular orbit. Its radius is neither perihelion, aphelion, semi-major, not semi-minor, so it wouldn’t have the same revolutionary period as Earth. Sorry, what you have is a fictoid.

  18. hhEb09'1

    I didn’t make unstated assumptions, and nothing you’ve contributed contradicts what I said. My posts are facts, whether I should or shouldn’t say them is a matter of opinion.

    The center of earth’s orbit is not the sun, anyone who objects to that has a problem with Newtonian physics. The center of an ellipse is different than the focus of an ellipse; for the earth, the focus lies within the sun–which is not necessarily the case, Jupiter’s is outside the sun.

  19. eyesoars

    I wonder what point on the earth was directly under the Sun during the perihelion…

    It will be near 22.5 degrees South, about 11 hours ahead of Greenwich, so about 165 degrees E.? Which would put it near New Caledonia in the Coral Sea (E. of Australia, NW of New Zealand).

  20. Kevin

    Perihelion Perihelion
    Happy Perihelion!
    Outbound from the Sun once again
    Hey Winter! Your time is surely at an end!

    You thought you would have your way
    But orbital mechanics has the final say.
    So take your cold and snow and ice
    And hit the road: we want the weather to be warm and nice. :)

  21. @ Zippy (#14): That’s not Jupiter. That’s Nibiru, and it’s headed our way.
    I read that in an email my Aunt Sally forwarded to me, so it must be true!

  22. ChazInMT

    @hhEbo9…Again, the Perihelion distance is 91,400,000 miles, the Aphelion is 94,500,000 miles. That means the difference is 3,100,000 miles. If you draw a circle with the Sun in the middle of the circle, it would need to be .28 inches wide.

    (((much study of axis of ellipses & such to be sure I’m not making a fool of myself)))

    OK, I suppose the orbit itself is quite circular as you point out, the Sun is just not in the center of the circle/ellipse. Indeed, if you look at the shape of the orbit Earth makes, it would fit into a very fine line on a piece of 8.5″ paper, as in, .002 inches, So I learned sumthin here. Also, the difference in radius on the ellipse is 21,000 KM, the radius of the Earth is 6,375 KM, so as others have mentioned the Earth does go outside itself, but certainly not by 3.1 million miles.

    So lets agree on this…Put a 1/16th inch dot on a piece of paper (Sun) then draw an incredibly fine line .002″ (couldn’t see it without a microscope) in a perfect 8.5″ circle centered 1/8th inch from the 1/16th inch dot.

  23. Ken

    Scott R @10: Coincidence. Both solstice and perihelion move by precession, but with different periods, so perihelion will eventually occur on every day of the (tropical) year.

  24. mike burkhart

    Hi I’m back on my new laptop I got for Christmas I love it Oh about Christoper Hitchens I know I’ve badmouthed him on this blog but when I heard he had passed away I pryed for him as I would pray for all who hate the Catholic Church or me .

  25. @ Mike (#25) Did you get a discount for buying a laptop with missing punctuation keys?

  26. Bill Nettles

    “I didn’t make unstated assumptions”
    Then what was the stated radius of your circular path in your original statement?

    “nothing you’ve contributed contradicts what I said.”
    After you make a clarification that wasn’t in your original comment. It was a poorly phrased statement if you intended it to be unequivocal. You also have a radius that no physicist who teaches this stuff would ever use.

    “The center of earth’s orbit is not the sun”
    Didn’t say that it was. I said the Earth orbits the Sun, which in Newtonian terms and Keplerian terms is what any reasonable physicist would assume. It’s an orbit around the center of mass of the system, and that is the more important center vs the geometrical center of an ellipse. And yes, I realize that the center of mass moves as the Earth orbits, but you work the problem using a reduced mass and fixed CM.
    ” anyone who objects to that has a problem with Newtonian physics.”
    Vice-versa. The geometrical center may not be the sun, but the gravitational and angular momentum center IS the center of mass of the system which is in the Sun, not too far from the center. Angular momentum about the center of the ellipse is NOT conserved, and that presents a problem for Newtonian physics, unless you believe that central forces don’t conserved angular momentum.
    ” The center of an ellipse is different than the focus of an ellipse”
    OK, that’s geometrical and obvious.
    “for the earth, the focus lies within the sun–which is not necessarily the case, Jupiter’s is outside the sun.”
    And your point is ?

  27. Chris A.

    @hhEb09’1 (#11):
    “The semi-major axis of the earth’s orbit is 149598000 km, and it’s semi-minor axis is 149577000 km, so splitting the difference between them means a little over 10,000 km, much less than the diameter of the earth.”

    I think the real problem comes down to this: To what circle are we comparing the ellipse? A circular orbit with the same period as Earth’s (whose orbital radius would be, according to Kepler’s Third Law, equal to the semi-major axis (a) of the actual orbit)? If we position this hypothetical circular orbit’s center at the center (not the focus) of the true (elliptical) orbit, we’d find that the maximum deviations between it and the actual orbit (at the ends of the minor axis) would, indeed, be 20,832 km, which is closer to twice the Earth’s diameter of 12,756 km than it is to the diameter.

    A circular orbit (again, concentric with the true orbit) which cuts inside the true orbit at the major axis by the same amount that it swings outside the minor axis would orbit faster than the Earth). Seems like a meaningless comparison, to me.

  28. hhEb09'1

    “Meaningless”? It shows you what Kepler was up against, at the time of his discovery. Of course he considered circles offset from the sun. His persistence, and Tycho’s superb data on the positions of Mars, revolutionized astronomy–and, eventually, physics.

    There were no “unstated assumptions” in my post. To find the circle that best fits an ellipse, with the least deviation from the circle, you use the center of the ellipse, and the average of the semi-minor and semi-major axis for the radius. You don’t have to assume anything, just calculate it from the known facts. If you do it any other way, you’ll get a worse fit. That’s a fact.

  29. Scott R

    Ken @24: thanks! I suspected as much but wasn’t sure. Always great to learn something new.

  30. Nigel Depledge

    @ hhEb09’1 (#29 et al.) –

    It seems to me that the problem stems from the fact that your comment about the difference between Earth’s actual orbit and your hypothetical circle comes in a thread that is about perihelion, and hence about the way Earth’s distance from the sun changes over the year.

    Within that context, people’s minds are already focused on the aphelion versus perihelion comparison, which is a difference of just over 3,000,000 km.

    It is therefore natural to assume that the circle to which you alluded would indeed be a circular orbit with a period of 365.25 days, and such a circle would deviate from Earth’s actual elliptical orbit by those distances others have stated. It is not obvious that the circular orbit to which you alluded was simply a circle centred at the centre of Earth’s elliptical orbit with a radius equal to the mean of the semi-major and semi-minor axes. If your hypothetical circle doesn’t have a period of 365.25 days, then what is the point of making the comparison? You might just as well state that Earth’s orbit has an eccentricity of only 0.02 (source: nineplanets.org).

  31. hhEb09'1

    @Nigel: I agree. Other people made assumptions. That often gets you in trouble, in science.

    I think that self-reflection alone is a good reason for making the comparison, although I didn’t anticipate such a strong reaction to a statement that everyone now seems to agree is true. I just thought it was an interesting, and little known, fact.

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