Many students confuse the causes of temperature variations on the planets. In particular, orbital eccentricity is often given far too important a role, and the way in which seasons occur is often ascribed to what are more properly called day/night or climatic zone effects. Here is a summary of various factors, in their approximate order of importance for the Earth, with links to more detailed discussions of some of the topics:
Distance from the Sun
— the average distance of a planet from the Sun (the semi-major axis of its orbit) is overwhelmingly the most important factor in determining its average temperature. Other than Venus, every planet is cooler than the next closest planet to the Sun, and even Venus would be much cooler if it were further from the Sun.
Atmospheric effects caused by winds and greenhouse gases and to a lesser extent, clouds
— extremely variable, since some planets have no atmospheres, and others have dense atmospheres. In general, the denser the atmosphere the more important its effects are. For the Earth, daytime temperatures are reduced more than a hundred degrees and nighttime temperatures increased by more than a hundred degrees compared with the temperature we would have if we had no atmosphere (like the Moon); and on Venus and the Jovian planets, atmospheric effects can rival (though they do not surpass) the effects of orbital size.
Latitude (climatic zones)
— on every planet the average path of the Sun is the same as the path that planet's Celestial Equator follows. At the Equator of a planet the Celestial Equator is vertical, passes through the zenith, and on those days when the Sun is on the Celestial Equator (on the Earth these are the Vernal and Autumnal Equinoxes) the Sun passes directly overhead, heating the surface more than when at lower altitudes (one square foot of sunlight hits one square foot of ground). At the Poles of a planet the Celestial Equator follows the Horizon, and when the Sun is on the Celestial Equator its light is greatly spread out. As you move from the Equator toward one of the poles of a planet, temperatures generally get lower and lower. For most planets this climatic zone effect (tropics versus temperate versus arctic zones) is substantially larger than seasonal effects, so that even during the polar summer it is colder than at the Equator; and depending upon how well winds distribute heat, climatic zone effects can be larger than day/night effects.
Rotation of the planet: day and night
— every planet rotates relative to the stars and relative to the Sun (we used to think that Mercury always kept the same face to the Sun, but that turned out to be wrong). The side of the planet which is facing the Sun has day, and the other side has night. If atmospheric effects are small, day/night changes are large, but if atmospheric effects are large, day/night changes are small or nonexistent (as on Venus).
Rotation of the planet: seasons
— Seasons are a variation in the heat received at a given location on a planet caused by the tilt of the planet's axis of rotation relative to its orbital motion. Any temperature change not caused by the tilt of the axis of rotation is not a seasonal variation.
The way in which seasons occur is that the Sun has a north/south motion relative to the planet's Celestial Equator which is equal to the tilt of the axis of rotation. If there is no tilt, the Sun has no north/south motion, always lies on the Celestial Equator, and follows the same path across the sky (at a given location) every day of the year. There may still be variations in weather such as those which occur on the Earth from day to day and week to week, but the large temperature variations from June to December which we refer to as seasons would not occur at all under those circumstances.
If a planet has a tilt, the Sun will move north and south during the year, causing warmer weather in the northern hemisphere and cooler weather in the southern hemisphere when it is north of the Celestial Equator, and cooler weather in the northern hemisphere and warmer weather in the southern hemisphere when it is south of the Celestial Equator. Summer and winter are always reversed in the two hemispheres, being summer in the hemisphere that corresponds to the Sun's position, and winter in the opposite hemisphere, so that the overall heating of the planet is unchanged (ignoring other factors). Spring and autumn are also always reversed, the temperatures being about the same, but the direction of temperature change being opposite.
Note that at the Equator of a planet there are usually no seasons in the sense being discussed here, for being in between hemispheres with opposing seasons, the Equator should also be in between in terms of temperature; and on the Earth, equatorial seasons are typically called "wet" and "dry" or some such description, rather than "summer" and "winter".
For planets with a moderate tilt such as the Earth, the north/south motion of the Sun is moderate, and although the poles are much warmer during summer than during winter (when they may have nearly half a year of night), they are still much colder than regions closer to the equator; and near the Equator there are no seasons, as already noted, because the Sun is up for about half a day every day of the year and is always high in the sky near "noon". But for planets with large tilts, such as Uranus and Pluto, the seasons can reverse the usual climatic zones, with the poles becoming warmer than the Equator ever does, as the Sun circles higher and higher, nearly to the zenith. Even in those cases, however, the effect is small compared to the orbital size effect, and the warmest temperatures on Pluto never exceed 350 degrees below zero Fahrenheit.
Final note about seasons: There are TWO reasons that it becomes warmer in the northern hemisphere as the Sun moves northward. (1) It is up longer, because stars closer to the pole stay up longer than stars closer to the equator; and (2) it is up higher. The latter effect mimics climatic zone effects, so that it is sometimes confused with those; but the former only occurs as a result of seasonal effects.
— on the Earth (and only on the Earth), which has both liquid (ocean/lake) and solid (continent/land) surfaces, temperatures vary less closer to water and more further from water, so that near the coast temperatures are lower during the day than inland, and higher at night than inland. No other planet has such effects, because no other planet has both liquid and solid surfaces.
— for most planets, the change in distance caused by their orbital eccentricity is too small to have any noticeable effect, and even for the three with relatively large eccentricities, the orbital effects are minor compared to the effects at the top of this list. On Mars, orbital effects are large enough to notice, but are still smaller than everything listed above, down to and including atmospheric effects. Only internal heating and geographical effects are smaller for Mars than the effect of its eccentricity. For Mercury, with a larger orbital eccentricity and no seasonal effects, there is an even more noticeable change in temperature due to its changing distance from the Sun; but day/night and climatic zone effects are still much larger than the effect of eccentricity. And on Pluto, even though its eccentricity is even larger, its large tilt produces seasonal effects much larger than the orbital effects.
Internal heat (from the deep interior, not heat stored near the surface by recent absorption of sunlight)
— some of the Jovian planets have considerable internal heat, which can affect temperatures even more than the distance from the Sun; but on non-Jovian planets internal heat is a relatively minor affair, and even for Neptune and Uranus internal heat is less important than many other factors listed above.
Summary for each planet of temperature factors in approximate order of importance:
Mercury — distance from Sun, day/night, climatic zone, eccentricity (no seasons)
Venus — distance from Sun, atmospheric effects
Earth — distance from Sun, atmospheric effects, climatic zone, day/night, seasons, geographical effects, eccentricity
Mars — distance from Sun, climatic zone, seasons, day/night, atmospheric effects, eccentricity
Jupiter — distance from Sun, atmospheric effects, internal heat, (no seasons)
Saturn — distance from Sun, atmospheric effects, internal heat, seasons
Uranus — distance from Sun, atmospheric effects, seasons
Neptune — distance from Sun, atmospheric effects, seasons, internal heat
Pluto — distance from Sun, seasons, day/night, climatic zones, eccentricity