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 is 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:
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 rival (but 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.
