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 time it takes for the Sun to drop below the horizon varies from about 15 degrees north or south of the equator to about 18 degrees north or south.
At the same time, there are two reasons why the sunset isn't exactly at the same time everywhere on Earth: first, because the rotation of the Earth is not uniform speed, but rather slows down as you go west (due to its interaction with the ocean), so night falls more slowly in western countries; second, because an object held at arm's length will be seen in relation to the distance between the object and the viewer, so the closer the object the faster it appears to move toward sunset or sunrise.
In conclusion, the time it takes for the Sun to drop below the horizon varies from place to place because of the difference in distance between the location and other places on Earth where sunset has already happened.
The Earth's orbit around the Sun is elliptical rather than circular, and the axis of rotation of the Earth is not perpendicular to the plane of the orbit. The non-circularity of the orbit, as well as the tilt of the Earth's axis of rotation, both contribute to the irregular shifts in dawn and sunset periods. The average distance between the Earth and the Sun is about 150 million km (93 million miles).
The length of a day is fixed at 1,392.9 minutes. But due to the ellipse shape of the Earth's orbit, there are times when we are closer to the Sun and therefore experience more sunlight during a day. During these orbits, the sun will have risen earlier in the morning and set later in the evening than expected given the date. The opposite is true during orbits where we are further from the Sun; here it will set later in the evening than expected given the date.
This is called an "annual cycle". The length of an annual cycle is approximately equal to the time it takes for the Earth to complete one full orbit around the Sun. This means that if we start counting days at midnight on January 1st, by the end of the year we will have counted up to 365 days has elapsed. However, because the Earth isn't exactly following a perfect circle around the Sun, some years are longer than others. A study of this phenomenon can be found in our next question.
The Sun looks to be in perpetual motion, rising on one side of the sky, travelling across it, and setting on the other. The apparent motion across the sky is caused by the Earth's rotation. Over the course of a year, these two motions impact the Sun's shifting location in the sky as well as the timings of daily sunrises and sunsets.
The earth orbits the sun but it takes 365.2422 days for the earth to make one trip around the sun. The period of time between two successive appearances of the same star or galaxy is called an eclipse. During a total solar eclipse, the Moon blocks out the sunlight coming from the Sun, causing darkness over a part of the planet for about an hour and twenty minutes. The angle at which the Moon comes between the Sun and the Earth determines how much of a shadow is cast on the ground; a full moon brings the entire disk of the Sun into shadow, while a quarter moon leaves a small crescent exposed below the horizon.
During a partial solar eclipse, some parts of the Sun are obscured by the Moon, while others are not. Westward-moving clouds or dust in the atmosphere can block out part of the Sun, causing local areas of darkness during a partial eclipse. Easternmost observers see all but the far western edge of the Sun covered by the Moon; westward-moving clouds or dust in the atmosphere can cause local areas of darkness within this zone.