So, if you measure the sun's height over the southern horizon at the same time every day, you'll see that it fluctuates. It is high in the summer and low in the winter. However, the Sun would travel quicker across the sky in January and slower in July, representing the variation in the Earth's motion around the Sun. The effect is so small that we can neglect it in our daily lives.
As the Earth revolves our Sun, the location of its axis in relation to the Sun varies. As a result, the observed height of our sun over the horizon changes. The Sun is seen to trace a higher route above the horizon in the summer and a lower path in the winter at any given place on Earth. This is because the axis about which it orbits is tilted with respect to the plane of the Earth's orbit around the Sun.
The apparent altitude of the Sun is a key factor in determining when plants will flower and when crops will be ready for harvest. Scientists use information on solar elevation to predict when and where there will be enough sunlight to grow crops profitably. They also use this information to determine if an area should be planted in wheat or corn, for example.
Elevation data are gathered by government agencies and commercial companies. For example, the National Oceanic and Atmospheric Administration (NOAA) gathers data on solar elevation every hour of each day throughout the year. These data can be found at http://eosweb.larc.nasa.gov/ssevents/.
Data on solar elevation are also collected by many satellite services. For example, the NASA-sponsored Solar Dynamics Observatory (SDO) sends back images of the Sun every few hours that show how high it is in the sky at that moment.
Because our globe is tilted on its axis, the sun's path through the sky changes throughout the year. The sun would rise and set later than usual in the spring and fall after the equinoxes, but sooner than usual in the summer and winter after the solstices. Days are getting longer as we move into spring, so there's no reason for the sun to set any later than it does now.
The earth's rotation is also affected by the presence of other planets. In particular, the axis of Earth's rotation is always held almost upright by the gravitational force of Jupiter, which prevents it from spinning completely around its axis. So even though days get longer as we move into spring, night won't necessarily come earlier; instead, the planet will just keep rotating at a constant speed throughout the year.
But this doesn't mean that spring comes late every time Jupiter is found in a different position in the sky. Sometimes it aligns with other stars or galaxies to create unusual astronomical events such as solar eclipses or supernovas. So although you can't predict when spring will arrive, you can look up in the sky and find out what's going on with respect to gravity fields all over the universe.
The amount of movement of the sun along your horizon at sunrise and sunset varies with the time of year as well as your latitude. Furthermore, the daily shift in position of the sun along the horizon is higher the farther north or south you are from the Earth's equator. All these factors combine to produce differences in both distance and direction between sunrise and sunset.
In general, the closer you are to the equator, the more dramatic the change in day and night lengths will be during the course of a year. For example, at the North Pole, the sun is below the horizon for three-fourths of the year, while at the South Pole it stays up for two-thirds of the year.
At the Equator, the sun rises due east and sets due west. So if you started out walking west toward the ocean, by the time the sun got high in the sky you would have walked far enough that you would be facing east instead (unless, of course, it was cloudy). The farther from the Equator you go, the more north or south you will be, which changes how much daylight and darkness there is each day. In the Northern Hemisphere, where it is light most of the time, people usually refer to this as the "morning" sun and the "evening" sun.
When the Earth is closer to the sun, as it is in January, our globe moves quicker in its orbit, covering more ground. From our vantage point, the sun appears to travel westward in the sky, requiring more than 24 hours to reach the same position from one day to the next (see here for a helpful image). The farther away from the equator we are, the faster this happens.
The effect of gravity on objects near the surface of the earth is called "geoid height". Geoid height is like the sea-level approximation to the earth's elevation above some arbitrary reference plane. It varies with latitude and longitude and is different at each location on the planet.
At the equator, where the force of gravity is equal all around us, sunrise and sunset occur simultaneously over most of North America and Europe because the horizon is even with the geoid. As you move north or south along the equator, however, sunset occurs before sunrise because the geoid is higher at the northern or southern latitudes. At the poles, where the force of gravity is greatest toward the center of the earth, sunrise and sunset occur simultaneously all year round because the horizon is always at geoid height there.
In conclusion, reason why sunset is earlier in the west is because the earth is closer to the sun in January.
The underlying process that leads in warmer temperatures in summer than in winter is a seasonal variation in the angle of sunlight induced by the tilt of the Earth's axis. Another influence is the shift in day duration. Both factors combine to lead to more solar radiation at higher latitudes in summer than in winter.
The amount of solar radiation that reaches the surface of the Earth varies throughout the year. At mid-latitudes, less radiation reaches the ground in winter because the Sun is lower in the sky then. In summer, more radiation reaches the ground because the Sun is higher in the sky then.
At the poles, night lasts from December to March, when the Earth is tilted away from the Sun. During this time there is no direct exposure to solar radiation, only indirect heat from the planet's core and ocean waters. In June and July, when the Earth is tilted towards the Sun, it becomes dark earlier in the day and light later at night. Solar radiation hits the ice and snow and warms them up.
As the snow melts, the water vapor in the air absorbs some of that radiation, which is why it is usually hotter in the summertime months. As the season progresses, the amount of solar radiation that strikes the pole increases, which causes the average temperature there during the summer to rise too.