Do days lengthen and shorten at a constant rate?

The earth undergoes a number of motions. The motion around the polar

axis accounts for the length of days and nights. The motion around the sun accounts for

the length of the year. Of interest here is the rotation of earth on the polar axis. The primary contributor to the differences we see in the length of periods of daylight is the tilt in earth's axis. You have noticed that globes of the earth are virtually always set on frames with the north-south pole oriented at an angle (of about 22.4 degrees). This is not because the tilt is more visually appealing. The earth is actually tilted that way, measured against the plane that contains the earth's whole orbit. When people in the south are experiencing summer around December-March, the south pole is tilted toward the sun. In the middle of the south temperate zone, more than half of your line of latitude will be in the sun at any moment. This is because of the tilt. As the earth rotates anyone at that latitude will spend more than half the day in sunlight. At the related latitude in the north, less than half the day will be in the sun. This all cycles back and forth as the earth orbits the sun, the axis maintaining its point almost exactly in the same direction.