Very basic astronomy question

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!

Slightly less variation in temperatures throughout the year? :confused:

It would change how the visible planets seem to move through the sky, but only slightly.

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?

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)

Here are a few differences:

http://curious.astro.cornell.edu/question.php?number=208

The Earth’s orbit is already darn near circular. Ref 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.

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).

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

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

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.

What is there to explain? Seasons are caused by the axial tilt. Seasons are affected by distance to the Sun. emproche

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).

People would no longer be able to make photographs like this. (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.

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.

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.

Sun size in summer and winter:

The Earth-Sun distance is approximately 147.5 million km at perihelion and 152.6 million km at aphelion.
Plugging those values into the expression for earth’s black body temperature, 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.

Good point, and here’s a page discussing it further.

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.