- June 21 was summer solstice, the longest day of the year — as compared to the night, in the northern hemisphere.
- June 21 was summer solstice, the longest day of the year — as compared to the night — in the northern hemisphere.
- Winter solstice occurs on December 21 or 22, when the night hours are the longest.
- The reason for different lengths of the day is because of Earth’s tilt.
- The planet’s axis of rotation is tilted at an angle of 23.5°C.
- Day occurs on the side facing the Sun, and changes to night as Earth continues to spin on its axis. On the Equator, day and night are equal.
- The closer one moves towards the poles, the more extreme the variation.
- During summer in either hemisphere, that pole is tilted towards the sun and the polar region receives 24 hours of daylight for months.
- During winter, the region is in total darkness for months.
The Celestial Sphere
- The celestial sphere is the apparent sphere of the sky.
- The Earth is at the center of the celestial sphere, so our view is always of the inside of the sphere.
- The celestial equator and poles are the projections of the Earth’s equator and axis of rotation out into space.
- The celestial poles are therefore located directly over the Earth’s poles.
How the Sun Moves on the Celestial Sphere? The Ecliptic plane and the ecliptic
- The plane of the Earth’s orbit around the Sun is called the ecliptic plane (See Figure a).
- As a result of the Earth’s annual motion around the Sun, it appears to us that the Sun slowly changes its position on the celestial sphere over the course of a year.
- The circular path that the Sun appears to trace out against the background of stars is called the ecliptic (See Figure b).
- As seen from Earth, the Sun appears to move around the celestial sphere along a circular path called the ecliptic.
- The Earth takes a year to complete one orbit around the Sun, so as seen by us the Sun takes a year to make a complete trip around the ecliptic.
- Further the ecliptic is inclined to the celestial equator by 231⁄2° because of the tilt of the Earth’s axis of rotation.
Origin of the Seasons
- As seen above the Earth’s axis of rotation is inclined 231⁄2° away from the perpendicular to the plane of the Earth’s orbit.
- The Earth maintains this orientation as it orbits the Sun.
- Consequently, the amount of solar illumination and the number of daylight hours at any location on Earth vary in a regular pattern throughout the year.
- This is the origin of the seasons.
- The ecliptic intersects the celestial equator at two points, called equinoxes.
- On about March 21 of each year, the Sun passes northward across the celestial equator at the vernal equinox. This marks the beginning of spring in the northern hemisphere (“vernal” is from the Latin for “spring”).
- On about September 22 the Sun moves southward across the celestial equator at the autumnal equinox, marking the moment when fall begins in the northern hemisphere.
- Between the vernal and autumnal equinoxes lie two other significant locations along the ecliptic.
- The northernmost point on the ecliptic is the summer solstice, and the southernmost point is the winter solstice.
- The point on the ecliptic farthest north of the celestial equator is called the summer solstice.
- It is at the summer solstice that the Sun stops moving northward on the celestial sphere. At this point, the Sun is as far north of the celestial equator as it can get.
- It marks the location of the Sun at the moment summer begins in the northern hemisphere (about June 21).
- At the beginning of the northern hemisphere’s winter (about December 21), the Sun is farthest south of the celestial equator at a point called the winter solstice.
Reason for longest and shortest days
- Because the Sun’s position on the celestial sphere varies slowly over the course of a year, its daily path across the sky (due to the Earth’s rotation) also varies with the seasons (See Figure Below).
- On the first day of spring or the first day of fall, when the Sun is at one of the equinoxes, the Sun rises directly in the east and sets directly in the west.
- When the northern hemisphere is tilted away from the Sun and it is winter in the northern hemisphere, the Sun rises in the southeast.
- Four important latitudes on Earth are the Arctic Circle (661⁄2° north latitude), Tropic of Cancer (231⁄2° north latitude), Tropic of Capricorn (231⁄2° south latitude), and Antarctic Circle (661⁄2° south latitude).
- These figure below shows the significance of these latitudes when the Sun is at the winter solstice and at the summer solstice.