a day of 24 hours.Īnd last, but not least, seasonal changes also result in changes in the Sun’s apparent motion across the sky. In the southern hemisphere, the situation is exactly reversed, with the South Pole experiencing a “midnight sun” – i.e. In the winter, the climate becomes generally cooler and the days are shorter, with sunrise coming later and sunset happening sooner.Ībove the Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year – up to six months at the North Pole itself, which is known as a “polar night”. During the summer, the climate warms up and the sun appears earlier in the morning sky and sets at a later hour in the evening. In addition to producing variations in terms of temperature, this also results in variations in the amount of sunlight a hemisphere receives during the course of a year.īasically, when the North Pole is pointing towards the Sun, the northern hemisphere experiences summer and the southern hemisphere experiences winter. the Earth’s axis is tilted 23.439° towards the ecliptic – this results in seasonal changes. Viewed from the celestial north, the motion of the Earth appears to orbit the Sun in a counterclockwise direction. This means that every four years (in what is known as a Leap Year), the Earth calendar must include an extra day. a sidereal year – to complete a single orbit of the Sun. With an average orbital velocity of 107,200 km/h (66,600 mph), the Earth takes approximately 365.256 days – aka. But combined with its orbital period (see below), a solar day – that is, the time it takes for the Sun to return to the same place in the sky – works out to 24 hours exactly. This means, in essence, that a sidereal day is less than 24 hours.
With a rotational velocity of 1,674.4 km/h (1,040.4 mph), the Earth takes 23 hours, 56 minutes and 4.1 seconds to rotate once on its axis. In essence, Pluto and Venus were sent spinning in the other direction by a large impact, while another struck Uranus and knocked it over on its side! In all cases, a large impact is believed to be the cause. Pluto also has a retrograde motion, so for those standing on its surface, the Sun would rise in the west and set in the east. Uranus is another, which not only rotates westward, but is inclined so much that it appears to be sitting on its side relative to the Sun. Venus is one exception, which rotates backwards compared to its orbit around the Sun (a phenomena known as retrograde motion). This is also true of the majority of the planets in the Solar System. This is true of all celestial objects observed in the sky, with an “apparent motion” that takes them from east to west.Įarth’s axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit. Because of this, to those standing on the Earth’s surface, the Sun appears to be moving around us in a westerly direction at a rate of 15° an hour (or 15′ a minute). If viewed from above the celestial north, the Earth would appear to be rotating counter-clockwise. Earth’s Rotation:Īs already noted, the Earth rotates on its axis as it circles the Sun. From this, we get the familiar passage of the Sun through the sky, and the basis for our measurement of time. The Sun only appears to be revolving around us because our planet not only orbits it, but also rotates on its axis as it is doing so. With the birth of modern astronomy, we have come to learn that its actually the other way around. Naturally, ancient people took the passage of the Sun through the sky as a sign that it was revolving around us. But have you ever wondered why the Sun behaves this way? Why does – and always has, for that matter – the Sun rise in the east and set in the west? What mechanics are behind this?
You may have heard the saying at some point in your life: “The Sun will still rise in the east and set in the west tomorrow.” You get the point, it means it’s not the end of the world.
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