# Very basic astronomy question **URL:** \ **Category:** Factual Questions\ **Created:** [February 11, 2008, 12:26pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144 "2008-02-11T12:26:54Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![brujaja](https://avatars.discourse-cdn.com/v4/letter/b/4af34b/32.png) [@brujaja](https://boards.straightdope.com/u/brujaja)\ **Post date:** [February 11, 2008, 12:26pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/1 "2008-02-11T12:26:54Z")
Hello all, I guess I really should know this, but I have trouble visualizing three-dimensional systems in motion. If the orbit of the earth were somehow rounded out, i.e., rendered less elliptical, this would change the length of the year, but not the length of days, right? What else would it likely do? Just curious, thanks! ---
**Author:** ![Pushkin](https://avatars.discourse-cdn.com/v4/letter/p/ee59a6/32.png) [@Pushkin](https://boards.straightdope.com/u/Pushkin)\ **Post date:** [February 11, 2008, 12:34pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/2 "2008-02-11T12:34:46Z")
Slightly less variation in temperatures throughout the year? 😕 ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [February 11, 2008, 12:58pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/3 "2008-02-11T12:58:55Z")
It would change how the visible planets seem to move through the sky, but only slightly. ---
**Author:** ![brujaja](https://avatars.discourse-cdn.com/v4/letter/b/4af34b/32.png) [@brujaja](https://boards.straightdope.com/u/brujaja)\ **Post date:** [February 11, 2008, 1:05pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/4 "2008-02-11T13:05:07Z")
What about the seasons? Would everything just be shorter – four shorter seasons, proportionate to the shorter year? Or, would a round instead of elliptical orbit simply distribute the same distance more equally, and not change the year length? ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [February 11, 2008, 1:18pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/5 "2008-02-11T13:18:09Z")
The earth’s orbit is pretty close to circular already – it’s not all that elliptical. Its eccentricity is only 0.0167 (0 = circle, with ellipses running fro m 0 to 1) > **[Orbital eccentricity](https://en.wikipedia.org/wiki/Orbital_eccentricity)** > > In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values between 0 and 1 form an elliptic orbit, 1 is a parabolic (escape orbit or capture orbit), and greater than 1 is a hyperbola. The term derives its name from the parameters of conic sections, as every Kepler orbit is a conic section. It is normally used for the isol... Here are a few differences: > [@](#): > > Hence, due to ellipticity alone, > > Local noon occurs progressively later between October and April. > Local noon occurs progressively earlier between April and October. > The maximum deviation between local noon and clock’s noon turns out to be around 8 minutes. > > Now let us combine the two effects and see what happens near winter solstice: Obliquity and ellipticity both conspire to make local noon later than clock’s noon near winter solstice. Hence, this shifts sunrise and sunset timings later (note that without this effect, sunrise will become later and sunset will become earlier). The overall effect is that sunset starts becoming later before actual solstice even though the day is becoming shorter. [http://curious.astro.cornell.edu/question.php?number=208](http://curious.astro.cornell.edu/question.php?number=208) ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [February 11, 2008, 1:18pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/6 "2008-02-11T13:18:52Z")
The Earth’s orbit is already darn near circular. Ref [http://science.nasa.gov/headlines/y2001/ast03jul\_1.htm](http://science.nasa.gov/headlines/y2001/ast03jul_1.htm) for a layman’s explanantion. If the orbit was magically circularized one day the differences would be invisible to anyone not keeping careful track with a stopwatch & a telescope. And no, the length of the year would not change at all. At least assuming your magical circularization didn’t add or subtract energy from the whole system. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [February 11, 2008, 2:35pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/7 "2008-02-11T14:35:24Z")
[QUOTE=brujaja] … this would change the length of the year, but not the length of days, right? [/QUOTE] As LSLGuy notes, there’s no reason for the length of a year to change (i.e. there’s no reason a circular orbit necessarily takes less time than an elliptical one). ---
**Author:** ![emproche](https://avatars.discourse-cdn.com/v4/letter/e/6a8cbe/32.png) [@emproche](https://boards.straightdope.com/u/emproche)\ **Post date:** [February 11, 2008, 3:02pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/8 "2008-02-11T15:02:15Z")
The seasons are not equally long now; summer in the Northern Hemisphere is about 93 days long, winter about 89 days. With a circular orbit all seasons would have the same length. Currently we are 91.5 M miles from the Sun early in January; 94.5 Miles early in July. A circular orbit of about 93 M miles would cause cooler winters in the north and warmer summers. emproche ---
**Author:** ![Turek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/turek/32/18782_2.png) [@Turek](https://boards.straightdope.com/u/Turek)\ **Post date:** [February 11, 2008, 3:59pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/9 "2008-02-11T15:59:59Z")
[QUOTE=emproche] The seasons are not equally long now; summer in the Northern Hemisphere is about 93 days long, winter about 89 days. With a circular orbit all seasons would have the same length. Currently we are 91.5 M miles from the Sun early in January; 94.5 Miles early in July. A circular orbit of about 93 M miles would cause cooler winters in the north and warmer summers. emproche [/QUOTE] Seasons are caused by the tilting of the axis, not the varying distance from the sun caused by the elliptical orbit. In the winter, less of the sun’s rays directly strike the earth than during summer. If it’s winter in the Northern Hemisphere(and by the elliptical theory of seasons, the earth is farther from the sun), how do you explain the fact that at the same time it’s summer in the Southern Hemisphere? [Cite](http://curious.astro.cornell.edu/question.php?number=196) ---
**Author:** ![K364](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k364/32/5_2.png) [@K364](https://boards.straightdope.com/u/K364)\ **Post date:** [February 11, 2008, 4:01pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/10 "2008-02-11T16:01:45Z")
The eccentricity does not cause the seasons - the tilt of the axis does. In fact, for the Northern Hemisphere the sun is at it’s nearest in January. It seems that the great oceans in the Southern H. have a moderating effect on the severity of the seasons. In the Northern H., with all the big land masses, that moderation is less. So, the eccentricity helps moderate in the North. I’m suggesting a circular orbit would make Northern H. seasons more severe. ---
**Author:** ![emproche](https://avatars.discourse-cdn.com/v4/letter/e/6a8cbe/32.png) [@emproche](https://boards.straightdope.com/u/emproche)\ **Post date:** [February 11, 2008, 4:13pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/11 "2008-02-11T16:13:50Z")
[QUOTE=Turek] Seasons are caused by the tilting of the axis, not the varying distance from the sun caused by the elliptical orbit. In the winter, less of the sun’s rays directly strike the earth than during summer. If it’s winter in the Northern Hemisphere(and by the elliptical theory of seasons, the earth is farther from the sun), how do you explain the fact that at the same time it’s summer in the Southern Hemisphere? [Cite](http://curious.astro.cornell.edu/question.php?number=196) [/QUOTE] What is there to explain? Seasons are caused by the axial tilt. Seasons are affected by distance to the Sun. emproche ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [February 11, 2008, 4:28pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/12 "2008-02-11T16:28:19Z")
A circular orbit has one radius, whereas an elliptical one has a range of radii. Where in our present range of radii you choose the value for your new circular orbit will dictate whether the new year is longer or shorter. Ditto all the folks pointing out that seasonal changes are practically caused by the tilt of our rotational axis relative to our revolutional axis. Solar eclipses could look a bit different. At present, the varying Earth-Sun distance and Earth-Moon distance conspire to change how completely the Moon hides the Sun. In an eerie coincidence, the ratio of Earth-Sun to Earth-Moon distance and the ratio of Sun diameter to Moon diameter are both about 400:1, and the range of the distance ratio extends on either side of the diameter ratio. For this reason, we can have solar eclipses where you see a bit of the Sun all the way around the Moon, and eclipses where you don’t. I know the OP asked about changing our eliptical orbit into a circular one, but Earth’s orbit also has a bit of 12-sidedness going on because Earth wobbles a bit in balance with the Moon’s orbit. Also, while trying to think about all this in 3D, you should consider that the Moon’s orbit is quite circular too - around the Sun, not around the Earth. The Moon’s distance from the Sun varies by +/- 1/4% as it does its dance with Earth. The Moon’s motion is everyplace concave towards the Sun, and almost half the time convex towards Earth. The Moon is influenced by the Sun’s gravity much more than by Earth’s (though the second order issue of the field gradient and resulting tides on the Moon is more dependant on the Earth than on the Sun). ---
**Author:** ![Bytegeist](https://avatars.discourse-cdn.com/v4/letter/b/9dc877/32.png) [@Bytegeist](https://boards.straightdope.com/u/Bytegeist)\ **Post date:** [February 11, 2008, 4:28pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/13 "2008-02-11T16:28:35Z")
[QUOTE=brujaja] What else would it likely do? [/QUOTE] People would no longer be able to make photographs [like this](http://www.analemma.de/images/articles/rumyants.jpg). (The figure eight would be “squished” into a line.) The rowdy celebrations we’re used to having on _Aphelion Day_ and _Perihelion Day_ would need to find different outlets. ---
**Author:** ![brujaja](https://avatars.discourse-cdn.com/v4/letter/b/4af34b/32.png) [@brujaja](https://boards.straightdope.com/u/brujaja)\ **Post date:** [February 11, 2008, 4:40pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/14 "2008-02-11T16:40:46Z")
[QUOTE=Bytegeist] People would no longer be able to make photographs [like this](http://www.analemma.de/images/articles/rumyants.jpg). (The figure eight would be “squished” into a line.) The rowdy celebrations we’re used to having on _Aphelion Day_ and _Perihelion Day_ would need to find different outlets. [/QUOTE] Oh, wow, Bytegeist! Awesome photo! And I looove your username. Thank you, all you erudite astronomically-inclined folks. I had really thought that our orbit was a more pronounced ellipse. ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [February 11, 2008, 5:00pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/15 "2008-02-11T17:00:33Z")
[QUOTE=Bytegeist] People would no longer be able to make photographs [like this](http://www.analemma.de/images/articles/rumyants.jpg). (The figure eight would be “squished” into a line.) [/QUOTE] Not quite–the Equation of Time arises from both axial tilt and eccentricity, and without the latter there would still be the former. With a circular orbit, the analemma would be _narrowed_ into a skinnier and perfectly symmetric figure 8. As for the seasons, its true of course that axial tilt is the primary cause, but eccentricity also plays a small part. Because of the inverse square law, we receive 7% more solar radiation in January than in July. Logically, this should moderate the seasons in the northern hemisphere and accentuate them in the south. In practice, this is hard to observe because of local variation. I’m sitting here in the northern mid-latitudes on a below-zero day, and there is nothing moderate at all about our seasons–because I’m in the middle of a huge land mass. On a global scale, having perihelion in January _does_ probably moderate the northern winters. This effect shifts over thousands of years as perihelion cycles through the seasons, and may play a part in long-term climate cycles such as Ice Ages. With a circular orbit, this source of variation would be lost. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [February 11, 2008, 5:01pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/16 "2008-02-11T17:01:49Z")
[Sun size in summer and winter:](http://antwrp.gsfc.nasa.gov/apod/ap070709.html) > [@](#): > > The angular size of the Sun is noticeably smaller during July, when it is farther away. If the Earth’s orbit was perfectly circular, the Sun would always appear to be the same size. [The Earth-Sun distance is approximately 147.5 million km at perihelion and 152.6 million km at aphelion.](http://epod.usra.edu/archive/epodviewer.php3?oid=196160) Plugging those values into the expression for [earth’s black body temperature](http://en.wikipedia.org/wiki/Black_body), T[sub]E[/sub] = (R[sub]S[/sub]/2D)[sup]1/2[/sup], yields temperatures of 289.5°K and 284.6°K for perihelion and aphelion, respectively. Ignoring greenhouse effects, that’s a 4.9°C, or 7.2°F, change in temperature which wouldn’t happen if the orbit was circular. ---
**Author:** ![Bytegeist](https://avatars.discourse-cdn.com/v4/letter/b/9dc877/32.png) [@Bytegeist](https://boards.straightdope.com/u/Bytegeist)\ **Post date:** [February 11, 2008, 5:34pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/17 "2008-02-11T17:34:31Z")
[QUOTE=Freddy the Pig] Not quite–the Equation of Time arises from both axial tilt and eccentricity, and without the latter there would still be the former. With a circular orbit, the analemma would be _narrowed_ into a skinnier and perfectly symmetric figure 8. [/QUOTE] Good point, and [here’s a page](http://solar-center.stanford.edu/FAQ/Qsunaspattern.html) discussing it further. ---
**Author:** ![Bytegeist](https://avatars.discourse-cdn.com/v4/letter/b/9dc877/32.png) [@Bytegeist](https://boards.straightdope.com/u/Bytegeist)\ **Post date:** [February 11, 2008, 5:37pm UTC](https://boards.straightdope.com/t/very-basic-astronomy-question/437144/18 "2008-02-11T17:37:40Z")
[QUOTE=brujaja] And I looove your username. [/quote] Well thankee kindly. I’m afraid it’s not entirely original — but in the end, it identifies me, and that’s all you can reasonably ask of a user name.