As the seasons vary, the sun reaches its greatest position at a variety of times, not just at noon every day. This is primarily due to the second major contribution to the Sun's apparent motion during the year: Earth's orbit around the Sun is elliptical rather than circular. During parts of each year, when Earth is in the Sun, it does not face the Sun directly but instead orbits along a line called an ecliptic. At other times, we call these times when Earth is in the Sun "solstices", since that is the only time when all points on Earth are directly facing the Sun.
When Earth is near perihelion (the closest point to the Sun), days are longer and temperatures are lower than when we are far from perihelion. As Earth moves farther from the Sun, days get shorter and temperatures rise. By the same token, when Earth is near aphelion (farthest distance from the Sun), nights will be longer and temperatures will drop low enough for ice to form again if it has been thawing out during the summer.
The amount by which Earth moves away from or toward the Sun is called its orbital radius. The average distance between Earth and the Sun is about 150 million kilometers (93 million miles). But because Earth's orbit is elliptical, it can be anywhere from 147 million km to 152 million km from the Sun from January 2 through July 2.
The Earth's orbit around the Sun is somewhat elliptical rather than totally round. Because of the Earth's eccentric orbit and the tilt of its axis, the Sun travels across the sky at somewhat different rates each day. As a result, we have varied dawn and sunset times each day. The amount of time that it takes for the Sun to rise or set varies from about 11 hours 39 minutes to 14 hours 19 minutes as the Earth orbits from perihelion (nearest point) to aphelion (farthest point).
This is called an "annual cycle". The axial tilt causes the rate of rotation of the earth to slow down as it gets closer to winter solstice (point in which the axis of the earth is most inclined toward the celestial equator) and speed up again as it gets farther from winter solstice. This is why summer and winter nights are longer when you are near the equator and shorter when you are far away from the equator.
The annual cycle also affects the length of day and night. At the summer solstice, the longest day of the year, the sun is high in the sky and low on the horizon for few hours before sunrise and after sunset. At the winter solstice, the shortest day of the year, the sun is low in the sky and remains there until just before sunrise and after sunset.
The average sunrise time on the summer solstice is about 5:30 a.m., for example, while the average sunset time on the winter solstice is about 7:00 p.m.
The Earth's rotation also causes some variations in sunset times from place to place. At the equator, where the sun appears to rise and set over the same spot, the sun will be at its highest altitude around 6:00 a.m. and 12:00 noon. The farther from the equator you go, however, the later it will be before it gets completely dark and the closer it comes to sunset. At the poles, where the sun appears to rise and set over different spots, there is no midday sun. Instead, there is a constant daylight period throughout the year that only ends when it gets dark outside.
The angle at which the Earth orbits the Sun determines how much sunlight reaches each part of the planet. On the far side, all sunlight must pass through the atmosphere before reaching the ground, so most of it is blocked out by clouds or reflected back up into space.
Because of the Earth's changing distance from the sun, the time it takes the sun to reach the same point in the sky each day in December. What was once called high-noon sunlight is now later in the month. Sunset is at least an hour later in the winter than in the summer.
This is because the solar day is about 24 hours long, but due to the rotation of the earth, daylight lasts only about 16 hours. At the equator, where the sun is most high in the sky during mid-summer, it's farthest away from the horizon and twilight ends around six o'clock in the evening. But at the poles, where the sun is low on the horizon during mid-winter, it's closer than it is to the horizon at midday and no twilight exists at all. The sun sets when its center is below the horizon. Since the earth is rotating, there are places where sunset occurs before the sun has actually gone down.
At the equator, the sunrise is about an hour after sunset. But at the poles, where the sun is low on the horizon early in the morning, it rises before dawn and sees the first light of day. The time between sunrise and sunset is short at the poles and can be as little as 30 minutes, but it can also be as long as 18 hours.