Close but still so far away – the sun at perihelion


Quadrantid meteor shower Jan. 2-3, 2013

Were you like me and got up this morning only to find the sky still overcast? No meteors for this poor astronomer. I hope some readers fared better.

It always seems to be clear over John Chumack’s home in Dayton, Ohio. Chumack, a long-time amateur astronomer, recorded 52 Quadrantid shower meteors overnight using a low-light video camera. Click the video above to watch it all go by in just 33 seconds.

Earth’s oval or elliptical orbit causes our distance from the sun and orbital speed to vary during a year. This week we’re both closest and moving fastest. Illustration: Bob King

At 10:37 p.m. January 1 this week, Earth passed an annual milestone in its orbit, reaching its closest point to the sun for the year. Astronomers call it perihelion, a Greek-rooted word combining ‘peri’ (close) and ‘helios’ (sun). Earth’s distance from the sun varies over the course of a year because our orbit is not a circle with the sun at the center. Rather it’s an ellipse – like all the other planets’ orbits – with the sun slightly off to one side.

On July 5 this year, Earth will reach its farthest distance from the sun called aphelion (AP-hee-lee-on). The difference from one side of our orbit to the other is only about 3 million miles or 3.3%. While the change in distance affects the amount of heat we receive from the sun, it’s not nearly enough to affect the seasons, which are caused by the 23.5 tilt of our planet’s axis. The tilt of the north pole toward the Sun in June causes summer north of the equator, while summer south of the equator comes six months later when the south pole is facing the Sun.

Difference in the size of the sun when Earth was at aphelion (top) last July and this week at perihelion. It’s very obvious in side-by-side photos but extremely difficult to discern with the naked eye. The difference amounts to just 1.1 minute of arc or 1/30 the diameter of the full moon. Credit: Giorgio Rizzarelli

Because our distance from the sun varies, so does the sun’s size and our planet’s orbital speed. When closest to the sun, Earth moves faster than when farther away, the same way sun-hugging Mercury orbits faster than distant Jupiter. Our average speed is 18.5 miles per second (66,600 mph) through space, but today we’re zipping along 2,160 mph faster than we will come July. I can almost feel the wind in my thinning hair.

The sun is peppered with sunspots in this photo made at  9 a.m. CST today Jan. 3 by the Solar Dynamics Observatory. Sunlight takes 8.2 minutes to arrive at Earth at perihelion and 8.5 minutes at aphelion. Credit: NASA

Giorgio Rizzarelli, a regular reader and commenter on this blog, performed an interesting experiment comparing the size of the sun at aphelion on July 5, 2012 and at perihelion earlier this week. The difference is immediately obvious from his unique perspective.

Giorgio went a step further and measured the difference in diameters to arrive at the Earth’s orbital eccentricity.

Eccentricity or ‘e’ refers to how stretched out a planet’s orbit is compared to a perfect circle. With a circle defined as e = 0, Rizzarelli calculated an ‘e’ of  0.017 (nearly circular but not quite) for Earth’s orbit, in excellent agreement with the published figure of 0.0167. (see calculation below). Amazing what you can do with a camera from your own backyard.

“The disc in lower photo is 3.4% bigger than in the upper, so (dividing by 2) 1.7% bigger than average. Hence Earth today is 1.7% closer to the Sun than average. This defines the approximately eccentricity, (which is) 1.7% or 0.017.” – Giorgio Rizzarelli

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About astrobob

My name is Bob King and I work at the Duluth News Tribune in Duluth, Minn. as a photographer and photo editor. I'm also an amateur astronomer and have been keen on the sky since age 11. My modest credentials include membership in the American Association of Variable Star Observers (AAVSO) where I'm a regular contributor, International Meteorite Collectors Assn. and Arrowhead Astronomical Society. I also teach community education astronomy classes at our local planetarium.

30 thoughts on “Close but still so far away – the sun at perihelion

  1. Hi Bob
    Thanks for the blog, is the Milankovitch’s theory just the same as that work’s out the same to be 0.017 eccentricity, maybe i’ve got that bit wrong, but the photo that Georgio sent you was amazing, seeing the difference in size it does look huge, but not, and does this have any effects on earth, might be a silly question but I need to ask. Thanks Bob, great start to the year, you just don’t know what’s round the corner in the world of space!

    • Hi Lynn, nice to meet you, thank you very much

      Yes eccentricity of Earth’s orbit has effects on climate. Said first that seasons are mainly due to Earth’s axial tilt, for coincidence the perihelion happens now, near solstice. In theory, having now sun closer, we at North hemisphere should have a moderate winter (respect South hemisphere which has winter in July, at aphelion). But really we at North have more extreme seasons because we have more dryland than South hemisphere. These effects are of few degrees.
      On Mars, which has big eccentricity and no sea, the difference of climate in the two hemispheres is dramatical: South summers are more extreme and have sandstorms.
      See details in this old Astrobob article:
      http://astrobob.areavoices.com/2009/01/03/a-minute-a-day-keeps-the-blues-away/

      PS If by Milankovitch theory you mean his famous cycles, about variations of the season features, yes, one of these cycles indeed consists in the variation in time of the eccentricity. However these variations can be appreciated only throgout epochs.

  2. re. meteor shower: well, the forecast was for mostly clear skies overnight, but nope! I even stayed up! A partially clear sky here might only last last 30 minutes this time of year. Looks like it was a pretty good show though. Thanks for sharing the video!

  3. Just curious: with Giorgio’s photo measurements and the calculations based on them, could the difference between his solution and the ‘published’ measurement be based on the seasonal tilt of the Earth? In other words, if he had taken the same second picture from the same latitude but in the opposite hemisphere (I assume in the southern hemisphere), would that make enough difference to have his numbers match exactly with the accepted published number?

    • Bob,
      That would be a very small difference – the tip might measure, what, a little less than one Earth radius or ~3500-4000 miles. That’s equal to 1/23,250th the average Earth-sun distance — pretty tiny. Let’s see what Giorgio says.

      • The other thing I thought about after writing that comment was that I had seen a web site somewhere that showed (or at least claimed to show) how much the different planets effect the Sun and cause it to ‘wobble.’ Jupiter, of course, seems to have a very big effect, causing the Sun to ‘wobble’ (if I remember right) about half of it’s diameter. That would be something like 500,000 miles, wouldn’t it? Could be a factor in the differences?

        • Bob,
          Fascinating observation! Yes, the sun orbits about the solar system’s barycenter (center of gravity for the entire solar system). Most of the tug on the sun is from Jupiter, but Saturn and the other planets also come into play. The total amount of displacement amounts to about one solar radius or 432,500 miles. That plays out to a 0.46% change in the Earth-sun distance (93 million miles) due to wobble induced primarily by Jupiter. It’s a small but significant amount that does come into play during the measure of each year’s perihelion and aphelion. Those distances change slightly every year due at least in part (maybe in whole) to where Earth happens to lie in relation to the sun’s position with respect to the barycenter. I didn’t address the perihelion-aphelion variation in the blog since it went beyond the scope, but your question offered a possible explanation for the variations I’ve noticed over the years.

          The next question is then: does the revolution of the sun around the barycenter affect eccentricity measurements like Giorgio’s? My hunch is that they balance each other out – being 6 months apart – so there’s no measurable change over many years.

          I suspect the difference in Giorgio’s value vs. the published ‘e’ may have more to do with the accuracy of his measurement compared to the high-powered tools and orbital computations professional astronomers use. His number is still very, very close considering he used a basic method like measuring diameters on photos.
          Earth’s orbital eccentricity does vary over a period of hundreds of thousands of years (range of .0034 to .058) due to the perturbing influences of the planets.

          • Hi Bob C, thank you for the interesting questions and nice to meet you. Sorry for the late in reply, it was night here in Italy.

            Astrobob is right. In a measure one never gets an exact value, but a value with an uncertainty, mostly because the instrumentation has limited precision: with 0.017 I really mean that the true value of Earth orbit eccentricity is a number which, rounded to 3 digits after the decimal point, gives 0.017. So my result is in agreement with 0.0167: there’s no discrepancy. It’s just that my experiment has a precision of 3 digits after point, that is, 1 part on 1000.

            Where does this 1/1000 uncertainty come from?
            I used a Celestron C8 telescope (with front solar filter, very important to avoid eye damage) and a Canon EOS 600D reflex camera body. In this configuration the Sun diameter occupies most of the long side of the picture, which is something like 5000 pixel. Luckily this matches perfectly the resolution of the telescope in this configuration.
            To measure each Sun disc in pixels I used a simple software for pictures. The uncertainty comes from the fact that the Sun border, if zoomed in, reveals to be blurred – because a brighter exposure “swells” Sun, air is not steady, focus and optics are imperfect … I estimated for this uncertainty about 5 pixels. 5/5000 is 1/1000.
            I uploaded here http://postimage.org/image/j6vq35m8v/full the pic in high resolution and here http://postimage.org/image/5ehb7ivhb/full/ an alternative form (joining slices of semicircles). You can do your own measure and precision estimate from the picture. Most likely using a C11 (which has more resolution) with another camera and a typical software to reduce the turbulence blurrings, one can get a better precision. On the other hand a friend of mine will repeat the experiment using a photocamera with teleobjective, he will get less precision but enough to measure an e>0. (continued in next comment)

          • (continued from previous comment)

            It’s important to avoid systematical errors, for example one must use exactly the same instrumentation at aphelion and perihelion, so that magnification power is the same. Another systematical error was, that here in Italy perihelion was on Jan 2nd while I photographed on Jan 1st (because 2nd was overcast), but I calculated that this systematical error is negligible respect the precision uncertainty of 1/1000.

            Now coming to the astronomical systematical errors which you quote, Astrobob knows quite more than me and, for what I know, I quote every words he wrote about. The changing of distance because of the axial tilt is certainly negligible. The perturbation by Jupiter is and interesting and complex matter. Does the one-Sun-radius (or 0.46%) recoil amplitude refer to a Jupiter year, I suppose? I can just say, with good sense, that in the 6 months of experiment Jupiter did just 1/24 of its orbit, so I don’t think Jupiter had an appreciable effect (it would be 0.46%/24=2/10000<<1/1000). Anyway, since I didn't find discrepancy to 1/1000 precision the effect must be smaller. What do you think? Possibly as Astrobob says, repeating the experiment for years, one would notice Jupiter's effect. I don't know if I will have the patience of old astronomers :)

    • Giorgio and Bob: thank you both for your replies!

      Giorgio, you did an amazing job with those two pictures of the sun and the measurement of the photos and the math; certainly amazing to me anyway! It is so cool to me to see how much we can learn about the bigger world around us just by doing reasonably straight forward things like that. I would never have thought of that!

      Giorgio, just a follow up question in regards to the possible influence of Jupiter on your results: last July, when we are the furthest from the Sun just because of our elliptical orbit, Jupiter was almost exactly behind the Sun from our perspective and so the Sun would be ‘wobbled’ (a good scientific term, eh?) even further away from us. And now, of course, we are almost exactly between the Sun and Jupiter and so the Sun should be ‘wobbled’ towards us right when we are in the closest part of our orbit anyway. Now, if Jupiter is having that big of an effect on the Sun, it must certainly be having an effect on Earth too, so how could we even begin to measure that?!

      Also, is Jupiter therefore the primary reason for our elliptical orbit with both the Earth and the Sun getting pulled towards it (at least when we are closest to it)? In other words, does our ellipse kind of follow the orbit of Jupiter (short side pointing toward Jupiter) or does our ellipse stay oriented in the same direction all the time regardless of where Jupiter is in its orbit?

      Man! Thinking about this gets me wondering about all kinds of things! If Jupiter (primarily) can make the Sun wobble by more than 400,000 miles, does it also have a measurable effect on things like ocean tides on Earth? Are the tides typically greater or lesser when we are closest to Jupiter?

      Sheesh, I can’t hardly wrap my mind around all of that!

      Thanks so much you guys!

      • Bob,
        I don’t think it’s quite as simple as Jupiter the sun being closer to us every time Jupiter is at opposition and farthest when Jupiter’s in conjunction. The period of the sun’s wobble also involves other planets. Since the Earth and Jupiter are in constant motion, my hunch is that only occasionally does it happen that the sun is slightly closer to Earth at the same time Jupiter’s in opposition.

        Our elliptical orbit does not follow Jupiter’s lead. If it did, the dates of perihelion and aphelion would be changing all over the calendar, not fixed as they are in January and July. Also, Jupiter has a negligible effect on ocean tides. Its gravitational effect on Earth is minimal because of its great distance. There are only two bodies in the solar system that routinely affect Earth in a measurable gravitational way on the scale of human history – the sun and the moon. They are the cause for the tides. Just to give you an idea – the sun’s effect on Earth’s tides is about the same as the much, much smaller moon’s. The moon’s influence only equals the sun’s because it’s so much closer. Jupiter and the other planets DO affect Earth’s orbital eccentricity over a period of hundreds of thousands of years and would also affect the tilt of our axis over time if it wasn’t for the moon.

        • Hmm…
          http://spaceplace.nasa.gov/barycenter/
          This animated diagram here is actually the opposite of what I was picturing in my head. I was thinking that the Sun would be pulled closer to us when when we (the Earth) are in between the Sun and Jupiter, and somehow that still seems to make the most sense in my head. But this NASA explanation would suggest the complete opposite. I have SO much to learn!

          • Bob,
            Yes, I’ve seen the same diagram. Both the sun and whatever planet you choose revolve around the barycenter, so our distance from it and therefore the sun remains constant until the barycenter shifts slightly with the changing positions of the planets. We then orbit that new barycenter at a constant distance from it and the sun. I think what’s important to remember is that the barycenter is inside the sun most of the time, so distance changes between Earth and sun are typically pretty minimal. It’s similar to the Earth-moon system, where the center of gravity between us and the moon is deep beneath Earth’s crust.
            And again, the sun doesn’t get pulled significantly closer to us when we line up with Jupiter because of the competing influences of the other planets.

          • @BobC: various questions here, some easy, others non-trivial. I’ll try to find the time to think about and write a relatively simple reply.

          • AstroBob already gave very good replies here, but on some points I’d like to add something. I’ll write back in a couple of days. Thanx again for the interesting questions.

        • Measuring the variation of Sun-Earth distance, like aphelion and perihelion, cannot reveal the effect of Jupiter or other planets. The reason is that Earth revolves around Sun (or better, along an ellipse with Sun in one focus), not around the SS barycenter. Writing here the theoretical reasons is a bit too specifical for here (@BobK: I’ll send you an email about). Anyway, facts show it. My same home experiment can be used to demonstrate it, as follows.
          If Earth revolved around SS barycenter, this year, since Jupiter’s opposition is roughly now at Earth perihelion, we’d had, respect average, a bigger perihelion distance and smaller aphelion distance, and so a smaller aphelion/perihelion ratio – while in about 2019, with Jupiter’s opposition at Earth aphelion, the ratio would be larger than average. So I’d had to repeat the experiment in 6 years, or (less honest method but faster) to just compare this year’s result with the value resulting from the published value of the eccentricity. The effect would be 0.46% (as said AstroBob). But my result has no discrepancy respect published value, at a precision of 0.001=0.1%.
          If instead of my experiment we look at the published values for aphelion and perihelion distances, it’s even more convincing: they are constant every year with a very high precision (152,097,700Km and 147,098,070 respectively).
          Hence Earth revolves Sun, and we cannot use such an experiment reveal effects of other planets.

          • Postscript: Considering all planets, instead of only Jupiter, don’t change concepts, nor orders of magnitude (Jupiter has the dominant effect).

            However, Sun’s actual motion due all to planets is very complicated because, when Newton’s laws are applied to a system of many bodies the result can become chaotic. This page of screenshots from a simulator shows it well:
            http://orbitsimulator.com/gravity/articles/ssbarycenter.html

          • Postscript2: The direction of axis of Earth’s elliptical orbit is basically fixed, as Astrobob says. Really it has indeed a precession caused by planets, but only on a period of an epoch. Perihelion precession (partly caused by other planets) is observed quite more for Mercury because it has a quite eccentric orbit.

          • Yes, indeed, thank you Giorgio. And thank you, too, Bob. I haven’t had lots of spare time recently, but I have been doing a little reading trying to put all of this away into my ‘keep-it-simple’ brain.

            I understand that our ellipse very slowly moves (turns) around the Sun. I think I read somewhere that it turns or rotates about 1.7% every 100 years (0.017% per year?). That is not much. With it being that steady, why does the precise time of perihelion and aphelion vary (back *and* forth) by as much as a couple of days from year to year? I have wondered the same thing about solstice (and equinox) times too.

          • heh heh heh… Our orbit around the moon? I was a little concerned for a minute! But I read that link and realized what you meant. It is because of our orbit around our Earth-Moon barycenter (there’s that barycenter ‘thing’ again!).

            I’m trying! I’m trying to get my head wrapped around this thing! But is *not* a ‘keep-it-simple’ thing!

            Would I be correct to say that it is the barycenter of a system (that mathematical point in space that is the center of gravity of that system) that moves *much more* than the dominant body of that system? And so when we make the barycenter a *fixed* center in a diagram, it makes it *appear* that the dominant body (the Sun or the Earth or whatever) is doing totally weird things like moving opposite to what it seems it should move?

            Again, thanks to both you and Giorgio for all the time you have given to this for me!

          • Bob,
            With Earth and moon revolving around the barycenter, it’s always moving. And yes, if you held it steady, then planets, etc. would make interesting loops, etc. around it.

  4. Hi Georgio, nice to meet you too and thank you for the explanation very much appreciated, so is the effect’s on climate small or can it be more harsh. Thanks and thanks to you too Bob for letting this go on your blog or if you can answer that question it doesn’t matter either from yourself or Georgio either one is appreciated :-)

    • As I wrote, regarding temperature differences between seasons on Earth, most effect comes from axial tilt, while Perihelion’s effect is of just few degrees (°C or °F) and covered by the effect, also of few degrees, of the land/sea distribution in the two hemispheres, which happens to be greater and opposite. All this contributes to natural Earth’s climate.
      Here’s another good article about http://science.nasa.gov/science-news/science-at-nasa/2000/ast30jun_1m/

  5. AstroBob, I just learned that similar experiments were performed, quite remarkably, in 1591-1604, without telescope, by no less than Tycho Brahe and, later and better, by (his assistant) the noted Johannes Kepler: they measured the apparent size of the Sun in different days of the year using self-constructed pinhole instruments, similar to a camera obscura (a term introduced by Kepler).
    Kepler’s instrument had a longer focal length than Brahe’s allowing more precision. His result was 31′ at perihelion and 30′ at aphelion, and he used the relative difference to get a first approximate measure of the eccentricity of Earth’s orbit. It’s remarkable that he obtained this evidence using the technology available at the time: the telescope was still not invented (it arrived just about 5 years after that). Kepler’s result was useful to him as a confirm to the value of 0.018 which he obtained for the eccentricity, of the Earth orbit, which he obtained by clever triangulation using Mars, when he was still thinking in the circular orbit approximation. This was a critical step in the development of Kepler’s laws.
    If interested in more info see:
    http://adsabs.harvard.edu/full/2001Obs…121..380S
    http://archive.org/stream/NewtonianMechanics/French-NewtonianMechanics#page/n588/mode/1up

    • Postscript: Sorry the first link didn’t work, just google “Measurements of the solar diameter in Kepler’s time”

    • Giorgio,
      Way to dig into the question. I’m always amazed by what Kepler and especially Tycho did before the invention of the telescope. Tycho was very smart to build his instruments large, which allowed for greater accuracy in determining planet positions. Very cool! Thanks Giorgio.

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