You are well aware that the brightness of today will in about 12 hours give way to the darkness of the night. You are also aware that there is a reversal of these conditions everyday – that while a part of the earth is lit up by the sun’s light, the other part, away from the sun, is swamped in darkness.
How and why does this happen? Is there an explanation for these phenomenon? Let us consider them.
The Earth’s Axis
The Earth’s axis is an imaginary line passing through the North and South Poles. The earth rotates around this axis from west to east, which is why the sun always appears to be rising on the eastern horizon and setting on the western. We cannot feel this motion (2200km/hr) because the earth moves smoothly and evenly at all times. Further, the sun does not move, it only appears so to an observer on earth, as the earth makes its rotation.
Also, it takes the earth 24 hours to make one complete turn on its axis, so an earth day is 24 hours long. The sun lights up one half of the earth, and the other half is in shadow. As the earth spins, we move from shadow to light and back to shadow and so on. It is daytime in Nigeria when our part of the planet is lit by the sun. And it is night-time when our part of the planet is facing away from the sun.
The Earth’s Tilt
The earth is tilted about its plane at an angle of 23.5° at the equator. For this reason, day and night are not evenly divided across the earth. If the Earth’s axis were at 90° to its orbital plane, all places on earth would experience equal amounts of day and night (i.e. 12 hours of day and night, respectively) every day during the year, and there would be no seasons – seasons become more noticeable as latitudes move farther away from the equator.
Instead, at any given time of the year, one hemisphere is pointed slightly more towards the sun, leaving the other pointed away. During this time, one hemisphere will be experiencing warmer temperatures and longer days, while the other will experience colder temperatures and longer nights.
The reason places in the northern hemisphere (US, UK, Russia, Japan, Sweden etc) are in autumn, while places in the southern hemisphere (Australia, New Zealand, South Africa, Chile, etc) are in spring.
Of course, since the Earth is rotating around the Sun and not just on its axis, this process is reversed during the course of a year. Every six months, the Earth undergoes a half orbit and changes positions to the other side of the Sun, allowing the other hemisphere to experience longer days and warmer temperatures.
Hence by December, the northern hemisphere will be in winter, and the southern hemisphere in summer. Consequently, in extreme places like the North and South pole, daylight or nighttime can last for days. Those times of the year when the northern and southern hemispheres experience their longest days and nights are called solstices, which occur twice a year for the northern and southern hemispheres.
Should you have any lingering questions, feel free to ask on our Comments section. We aim to please.
See Also: Understanding Time Zones: Why is it 3pm in Lagos, but 6am in Los Angeles?
