Online Astronomy eText: Orbital Motions / The Planets
Orbital Effects on Planetary Weather
(several variations on a theme, so somewhat repetitive)
(critical point: seasons are not caused by changing distance from the Sun)

The Relationship Between Semi-Major Axis and Average Temperatures
      All other things being equal the semi-major axis of a planet's orbit, which represents the average distance of the planet from the Sun, is the single most important factor in its average temperature and weather. Almost without exception planets that are closer to the Sun are warmer than those which are further away. Only Venus violates this relationship, as a result of an extreme ("runaway") greenhouse effect caused by its thick carbon dioxide atmosphere.
      It is reasonable that distance from the Sun should strongly affect temperature, because the apparent size and brightness of the Sun drops off as the inverse square of its distance. A planet 2 AU from the Sun would only receive, on the average, 1/4 as much light per square foot as the Earth does, at its distance of 1 AU from the Sun. Depending upon how the planet radiates the heat that it absorbs this would decrease its temperature by about 40%, measured on an absolute temperature scale (a temperature scale where absolute zero -- approximately 460 degrees below zero, Fahrenheit -- is the basis of temperature measurement). Since the Earth averages about 50 degrees above zero Fahrenheit (averaging day and night, summer and winter, Poles and Equator), or 510 degrees above absolute zero, a decrease of 40% would reduce its temperature by 200 degrees, to around 150 degrees below zero Fahrenheit. And this is about the temperature that one would expect for most objects in the region 2 AUs from the Sun, within the inner asteroid belt.
      Of course various complicating factors, such as mentioned above for Venus, can strongly affect the average temperature and deviations from that average, but if only the distance was involved the temperatures of the planets would vary according to the inverse square root of their semi-major axes. The diagram below shows how this would work, as well as the actual temperatures of the planets. The diagonal lines represent a reduction in temperature according to the inverse square root of the distance from the Sun, and the various dots, labeled with letters corresponding to the various planets, indicate the average temperature and distance of each planet from the Sun. As you can see there are significant variations in the planetary temperatures due to factors other than distance, which make some planets cooler than the overall trend lines and others warmer, but only Venus falls far outside the overall pattern.

Average planetary temperatures as a function of distance from the Sun
As distance from the Sun increases, temperatures generally decline

Effects of Orbital Eccentricity: The Earth
      Since, at least to a first approximation, planetary temperatures vary according to their distances from the Sun, we might also expect that temperatures would increase as a planet moves toward the Sun and decrease as it moves away from the Sun. If the distance variation, as expressed by the eccentricity, is small, the change in temperature might not be big enough to notice, but a large change in distance, corresponding to a large eccentricity, should provide easily observable effects.
      In the case of the Earth the small orbital eccentricity (only 1.7%) works against detecting any changes, but the fact that we live on the Earth makes it easier to detect changes, so we might hope to be able to see changes in the weather during the course of the year as we approach or go away from the Sun. As it turns out, however, there are a number of complicating factors for the Earth, two of which, seasonal variations in temperature and topographical variations, prove to be more important than the eccentricity, by factors of about 20 and 2, respectively.
      To see how this works, consider the fact that the Earth's distance from the Sun varies by about 3 million miles, from only 91 1/2 million miles when near perihelion in early January, to around 94 1/2 million miles when near aphelion in early July (the actual dates of perihelion and aphelion vary from year to year, according to when the most recent leap year was, and where you live relative to the International Date Line, but at the moment they are just a few days after the beginning of the respective months). With a change in distance of 3 million miles, you might think that there would be very noticeable changes in the temperature and weather, but that is not so, as is easy to see if you realize that in January, when we are closest to the Sun, most of the world's population is experiencing the cold temperatures of a northern hemisphere winter. Similarly, in July, when we are furthest from the Sun most people, living in areas turned toward the Sun by the tilt of our axis, are experiencing the warm weather of early summer.
      Now at least theoretically, the changing distance does produce a change in the amount of heat that we receive from the Sun. It is just that the expected change, in percentage terms, is only about twice the change in distance in percentage terms, and that is relatively small, despite the 3 million mile variation in distance.

Recent Questions and Replies: Feb 9, 2003:
How do the semi-major axis and eccentricity of an orbit affect the temperature and weather on a planet?

      The most obvious effect is that the closer the planet is to the Sun (the smaller its semi-major axis) the warmer it should be, all other things being equal, and in fact if you look at the surface temperatures in the
Planetary Data Table, you will find that with the exception of Venus that holds pretty well true.
      If the eccentricity is small, as it is for the Earth and all but three of the other planets, then the weather is hardly affected by the change in distance during one orbit. For the Earth this is made obvious by the fact that we are coldest in January, not only for the northern hemisphere but even for the Earth as a whole, when we are closest to the Sun (which occurs around the 3rd to the 5th of January, depending upon the most recent leap year and where you live relative to the International Date Line), and warmest in July, not only for the northern hemisphere, but for the Earth as a whole, when we are furthest from the Sun (around the 4th to the 6th of July).
      The fact that people living in the northern hemisphere are colder/warmer when near perihelion/aphelion is primarily due to the seasons, which are caused by the tilt of our axis (this is primarily part of essay question 2). The fact that the Earth as a whole is ALSO colder/warmer when near perihelion/aphelion is due to the difference in geography between the two hemispheres (mostly water in southern hemisphere, water and land in the northern hemisphere). In numerical terms, the tilt of our axis produces almost 20 times the effect of our changing distance from the Sun, and the difference in local geography is 2 to 3 times more important than the changing distance.
      This is only true for planets with small eccentricities, however. On Mars, with a 9% eccentricity, although whichever hemisphere is tilted towards the Sun has summer (regardless of its distance from the Sun), the perihelion summer is noticeably warmer than the aphelion summer, and has stronger and more widespread dust storms. On Mercury it is over 200 degrees warmer at perihelion than at aphelion, an effect easily measurable even from the Earth. Even on Pluto, where the temperatures don't vary as much because it is always close to absolute zero, it is about twice as warm relative to absolute zero at perihelion than it is at aphelion, and as a result, at perihelion methane and other volatile ices evaporate, forming a very thin temporary atmosphere. When Pluto moves away from the Sun and the temperatures drop to over 400 degrees below zero on the day side and over 450 degrees below zero on the night side, the thin atmosphere freezes and the planet no longer has any atmosphere.

     The following is a compilation of notes concerning the effect of orbits on planetary weather, from 2004/2005. They will be replaced by a more thorough textbook-style discussion when time permits.

Orbital Effects on Planetary Weather (1 of 2)
      Discussing the ‘weather’ (primarily means the temperature) on the planets, the MAIN THING that determines what is going on is the distance from the Sun (semi-major axis = average of the perihelion and aphelion distances). Planets that are close to the Sun are hotter than planets that are further away. In fact, with the exception of Venus, every single planet is cooler than the planets inside it (closer to the Sun).
     (refer to discussion above of changes on Mercury and Pluto)
     For Earth, in early January, we are 91 1/2 million mi (from Sun)
     In early July, we are 94 1/2 million mi (from Sun)
     AVERAGE = 93 million mi = semi-major axis (defined as 1 AU)
     Actual distance varies is 93 million plus/minus 1 1/2 million, depending on position in the orbit
     Or, as 1.00 AU plus/minus .017 AUS
     OR, as (first number) plus/minus 1.7 %
     This last way of doing things:
     FIRST NUMBER = size of orbit
     SECOND ONE = eccentricity (as a %)
     That % determines whether the temperature and weather on a planet are affected by changes in its distance from the Sun. For the Earth, the distance changes by 3 million miles, BUT THAT IS NOT IMPORTANT, as we can tell by the fact that it is winter in January and summer in July.
     That’s because the tilt of our axis causes a change of 50% plus/minus at mid-latitudes, whereas the eccentricity only causes a change of 3.4% plus/minus. SO the seasonal effect due to the tilt is about 15 or 20 times bigger than the effect of our orbital eccentricity, and is swamped by it.
     Of course, in the Southern Hemisphere, things are reversed -- it’s summer in January and winter in July -- so you might expect that in the Southern Hemisphere summers are warmer and winters are cooler than in the Northern Hemisphere. But you would be wrong, because there are other effects which are even more important -- especially where you are relative to a coast-line.
     In or near the ocean temperatures tend to remain roughly constant from day to night because sunlight goes down into the ocean quite a few feet and water requires a lot of heat to warm up, even a little bit. ON LAND temperatures go way up and down because the sunlight is absorbed at the surface, and since rocks/dirt are poor conductors, only the top few inches are heated up much, and so if you look at the evening weather reports, you see that inland things heat up during the day and cool off at night far more than near the coast.
     In the Northern Hemisphere, half the ‘earth’ is water, and half is land.
     In the Southern Hemisphere, it’s about 10% ‘earth’, and 90% water.
     Plop down at some random point in the North and you might be dry; in the South, you will almost certainly be wet.
     The fact that the Southern Hemisphere has far more ocean makes the average temperatures in the south more uniform than in the Northern Hemisphere. SO when we are closest to the Sun, the northern hemisphere is colder because of being tilted ‘away’ from the Sun (same tilt as always, but the Sun is in the wrong direction), and the southern hemisphere is warmer because it’s tilted toward the Sun AND because we’re pretty close, but it doesn’t get as much warmer, for either reason, as the northern hemisphere gets cold, because the northern hemisphere has more land.
     AS A RESULT, for the Earth as a whole July is the warmest month (even though we are furthest from the Sun), and January is the coldest (even though we are closest to the Sun). The warmer temperatures caused by the Northern Hemisphere summer in July overwhelm the colder temperatures caused by the Southern Hemisphere winter.

     How eccentricity affects temperature/weather on the planets. As discussed for the Earth, which only has an eccentricity of 1.7%, there is no observable effect. The seasons caused by our tilt are about 20 times more important, and even the difference in geography between N and S Hemispheres is more important than the eccentricity, so that the Earth as a whole is warmer in July when we are furthest from the Sun, and coldest in January when we are closest to the Sun.
     But on Mercury, where there aren’t any seasons (no tilt) and the eccentricity is very large (over 20%), the changing distance does produce a noticeable effect -- namely at perihelion, the temperature on the day side of the planet may exceed 800 Fahrenheit degrees, whereas at aphelion it rarely reaches 600 Fahrenheit.
     And on Pluto, where (although there are seasons) the eccentricity is 25%, at perihelion the temperatures can be as much as 350 below zero Fahrenheit, whereas at aphelion they can be more than 400 below zero. This produces a thin temporary atmosphere to form as Pluto approaches the Sun (it was closest in 1989 and closer to the Sun than Neptune until 1999), which freezes and falls to the ground as ices as the planet moves away from the Sun.
     For Mars, both the changing distance and the seasons are important. Mars has a tilt about the same as the Earth (25 degrees, compared to 23 1/2, for us), so it has seasons very similar to ours except twice as long, because its year is twice as long, and considerably colder, because its further from the Sun.
     As on the Earth the tilt on Mars produces, at mid-latitudes, changes of 50%, plus/minus, from the average amount of heat.
     Now on the Earth, the eccentricity only changes the amount of heat by 3.4%, plus/minus, so depending upon which hemisphere you’re in you’re going to expect a little more or a little less than a 50% change in solar heating, which is just about the same as what you expect from the seasons, so we don’t see any effect.
     But on MARS, you get a 20% change due to the change in distance (almost 10%), so for Mars if you are at perihelion during the summer and aphelion during the winter, the change in heat is: 50% + 20% = 70% more heat / less heat
     BUT if you are on the other side of the planet, where perihelion is during the winter and aphelion during the summer, these factors oppose each other and the change in heat is: 50% - 20% = 30% more heat / less heat
     In other words, the seasons should be twice as extreme in the hemisphere which has summer at perihelion than in the hemisphere which has winter at perihelion.
     For the winter it really doesn’t make any difference -- the Sun is down, so who cares how far away it is? On both winter hemispheres the temperature drops to 200 below zero Fahrenheit, at which point the atmosphere (carbon dioxide) begins to freeze out as dry ice snow. Eventually, half the winter hemisphere is covered by a few inches to a few yards of dry ice. IF THE WINTER WERE LONG ENOUGH (as it is on Pluto), eventually all of the atmosphere would move around to the winter side and freeze out, and Mars would temporarily become airless. But the year on Mars is too short for that to happen, and only about 1/3 of the atmosphere is lying on the surface as dry ice snow at the peak of the Martian winter. As the planet moves around the Sun and winter turns to summer the dry ice evaporates, the gas that results, along with the rest of the atmosphere, heads for the other side of the planet, and starts forming a polar cap on the other side.
     So you see polar caps forming and disappearing on alternate sides of the planet during a Martian year.
     Now, on the winter side the air is freezing and falling down, so there’s not a lot of it. On the summer side the previous winter’s polar cap is evaporating, so there’s more air. And where there is more air there is more pressure (which is equal to the weight of the air), and the higher pressure (about twice as high) on the summer side causes winds to blow toward the winter side. The large pressure difference causes strong winds at times, and these winds can kick up dust storms as big as counties, which are visible from the Earth as small yellowish spots.
     During the aphelion summer the heat of the summer side is only 30% more than normal. (Normal means daytime temperatures of about 20 to 40 below zero Fahrenheit.) This brings temperatures up to just above freezing.
     On the perihelion summer side, you get 70% more heat than normal, and temperatures may soar to 60 or on rare occasions, 70 degrees above zero.
     (Having landers on the surface and spacecraft in orbit, we can easily measure these Martian temperature differences.) On the Earth this does not happen, for two reasons: Our eccentricity is much smaller (1.7%, compared to almost 10%), and we have oceans and land surfaces with difference responses to the change in heat, whereas Mars is all dry land.
     There are still summer and winter on the appropriate hemispheres, regardless of whether Mars is at perihelion or aphelion. The tilt of 25 degrees is far more important than the eccentricity of only 10%. BUT the eccentricity is big enough to produce a noticeable modifying effect.
     In addition to the change of temperature, there is a noticeable effect on the weather, in that the dust devils and dust storms that occur on the summer hemisphere are relatively rare at aphelion, and much more common at perihelion. In fact at many perihelion summers, half the planet may be covered by dust storms, and on rare occasions the entire planet may be completely obscured.

Orbital Effects on Weather (2 of 2) (from second summer class; partially repetitive)
     Discussed the effects of orbits on weather (primarily on temperature), reviewing the effect of the semi-major axis and discussing the effect of eccentricity for four planets -- the Earth, Mercury, Mars, and Pluto.
      Earth’s orbital eccentricity is 1.7%
      In January we are 1.7% closer to the Sun than on the average (March and September), and the Sun looks 1.7% bigger in two directions (sideways and up/down), or 3.4% bigger (approximate math), and as it happens each part of the Sun looks equally bright, to a first approximation, so looking 3.4% bigger makes it 3.4% brighter, and it should be warmer in January than on the average.
     In July we are 1.7% further from the Sun than on average, and the Sun looks (1.7% one way, 3.4% total) smaller and fainter than usual, and it should be cooler.
     But that's wrong, because the tilt of the northern hemisphere away from the Sun in January reduces the heat we receive (at mid-latitudes) by 50%; and in July the tilt of the northern hemisphere towards the Sun increases the heat we receive by 50%.
     And 50% is a lot bigger than 3.4%, so the SEASONAL effects caused by our tilt completely swamp the orbital effects.
     Of course, the seasons are opposite in opposite hemispheres, so in January it is winter in the North, but summer in the South. So in the South, the 50% and 3.4% add together, instead of as in the North, opposing each other. This means that the seasons still agree with the tilt you have; but the seasons in the South should be ‘bigger’ than in the North (47% more/less heat in North, vs 53% in South). So you might expect to see an orbital effect by having more extreme seasons in the Southern Hemisphere. WHICH YOU DON’T, because there are lots of other effects as well, and one in particular -- the local geography of the two hemispheres -- swamps the 6% difference above.
     Land (1/2 land, 1/2 ocean in N) heats up a lot more in sunlight than ocean (90% ocean, 10% land in S), because (1) water needs a lot of heat to warm up, and (2) sunlight is absorbed only at the surface on land, but by tens of feet in the ocean. If you live on or near the ocean the average air temperature is relatively stable. If you live well inland the average air temperature is far more variable.
     Because of this, the 6% difference in ‘supposed-to-be’ temperatures between N and S is almost exactly reversed, with the N hemisphere getting far warmer in its summer than the S hemisphere does. So the Earth as a whole is actually warmest when at aphelion, furthest from the Sun, because that’s when the ‘land’ hemisphere in the N has its summer; and is coldest when at perihelion, closest to the Sun, because that’s when the ‘land’ hemisphere in the N has its winter.
     FOR MARS, because the tilt is similar to ours, you get about a 50% variation (at mid-latitudes) in solar heat because of the seasons. BUT because the eccentricity is almost 10%, the variation in heat due to the distance change is around 20%.
     So, at perihelion summer is 50% warmer + 20%
     And at aphelion summer is 50% warmer - 20%
     Only 30% more heat at aphelion summer, but 70% more heat at perihelion summer.
      On the winter hemisphere there is not much difference (if the Sun’s not up much, who cares how far away it is). The temperature on both winter hemispheres (perihelion and aphelion) drops to over 200 degrees below zero Fahrenheit, and would get colder yet except the air starts to freeze, and dry ice snow rains down on the winter hemisphere, covering half of it with up to several yards of dry ice. Up to 1/3 of the Martian atmosphere may be on the ground as dry ice snow at the peak of winter.
     Now, if you are freezing air out as snow on one side of the planet, and evaporating snow to make air on the other side, then there will be more air on the summer side, so there is more air pressure, which causes strong winds, which create large dust storms.

Weather On The Planets (Part 3; Spring 2005; again, somewhat repetitive)
     Distance from Sun (semi-major axis) is the most important factor in a planet’s temperature.
     IF A PLANET HAS SEASONS, that is usually the next most important factor (increase or decrease of heat during the year because of the Sun’s apparent N / S motion, caused by the tilt of the planet’s axis of rotation)
     ECCENTRICITY is of no importance if it is small (5% or less), and even for the three planets that have large eccentricities, eccentricity is only more important than seasons for one--Mercury (because it doesn’t have seasons). For Mars and Pluto, although eccentricity is important, seasons are more important.
     91 1/2 million miles from Sun in January
     94 1/2 million miles from Sun in July
     Average = 93 million miles = 1 AU
     Variation of 1 1/2 million miles (close/far)
     Is only 1 1/2 / 93 = 1.7% of average
     Produces a heat/light difference of 3.4%
     IN JANUARY, being closer to the Sun, it is 3.4% brighter. BUT in the northern hemisphere you get about 50% less heat than usual because the Sun is up lower and less than on the average.
     50% >> 3%
     50% less heat (seasons) + 3% more (distance) = 47% less heat than on the average (equivalent to moving closer to the Equator)
     IN JULY, 50% more heat (seasons) - 3% less (distance) = 47% more heat than on the average
     Of course,in the southern hemisphere the seasons are reversed,so
     50% less heat (winter) - 3% less = 53% less heat
     50% more heat (summer) + 3% more (closer) = 53% more heat
     You’d therefore expect South to have more extreme seasons (though by a teeny amount)
     However, there are other factors including climatic zones (warm tropics, cold poles), local geographical differences (water vs land)
     Water heats up / cools off slowly, land heats up / cools off quickly
     N hemisphere is 50% land 50% ocean
     S hemisphere is 10% land 90% ocean
     In January, N hemisphere has winter. The oceans cool a little, the land a lot.
     Meanwhile, the S hemisphere has summer. The oceans heat a little, the land a lot.
     BUT THERE’S A LOT MORE LAND IN N, so the Earth as a whole is a few degrees cooler, EVEN THOUGH CLOSER TO SUN.
     In July, N hemisphere has summer. Oceans heat a little, the land a lot
     Meanwhile, the S hemisphere has winter. Oceans cool a little, the land a lot.
     BUT THERE’S MORE LAND IN N, so the Earth as a while is a few degrees warmer, EVEN THOUGH FURTHER FROM SUN.
     So changing distance is important.
     At perihelion, distance is .3 AUs, more than 3 times closer than Earth
     Sun is 3 times bigger sideways/top = 10 times area and 10 times the brightness.
     Temperature more than 800 Fahrenheit.
     At aphelion, distance is .5 AUs, only 2 times closer than Earth
     Sun is 2 times bigger side/top = 4 times area, 2 1/2 times LESS than at perihelion
     Temperature is 500 to 600 Fahrenheit. This is MORE THAN 200 degrees change
     No physical change? Probably not.
     Pluto has 25% eccentricity.
     At perihelion, it’s 30 AUs from Sun, receiving 1/900 heat per square foot as Earth
     At aphelion, it’s 50 AUs from Sun, receiving 1/2500 heat per square foot as Earth= 1/3 as much as at perihelion
     At perihelion temperature is less than 100 degrees above absolute zero
     At aphelion temperature is less than 50 degress above absolute zero
     At perihelion, nitrogen and methane ices evaporate and form a VERY THIN temporary atmosphere.
     At aphelion, these gases refreeze.
     On Mars, eccentricity is 10%
     Tilt is similar to Earth (25 degrees)
     At perihelion summer, there is 50% more heat (summer) + 20% more (distance) = 70% more heat
     At aphelion summer, there is 50% more heat (summer) - 20% less (distance) = 30% more heat
     WINTER--who cares? Sun’s not up much
     Average daytime temp on Mars = -25 Fahrenheit (night -125)
     Winter temp = -210 Fahrenheit, and atmosphere freezes as dry ice snow, so 1/2 winter hemisphere has polar ice cap
     Summer aphelion 30% more heat, temp up 40 to 70 degrees from average, to 15 to 45 degrees above zero Fahrenheit
     Summer perihelion 70% more heat, temp up 70 to 100 degrees, to 45 to 75 degrees above zero
     On Mars, you tend to strong winds at some times, circulating heat from Equator to Poles or from the summer to winter side
     Speed of winds depends upon how thick the air is
     On Venus wind velocity is low but because the air is thick the temperature is relatively uniform
     On Earth wind velocity is higher but because the air isn’t as thick the temperature is more variable
     On Mars wind velocity is higher yet but because there is very little air there, the temperatures are even less uniform
     THE STRONG WINDS KICK UP DUST STORMS as big as counties, and as much as 30 miles high
     At aphelion summer you get a few dozen such major dust storms at any given time
     At perihelion summer you get hundreds of such storms and often they merge and grow and cover large parts of the planet (1/3 of summers about half the planet, 1/10 of summers all of it)