Sun Earth Relationship

The earth receives almost all its energy from the Sun"s radiation. Sun also has the most dominating influence on the changing climate of various locations on Earth at different times of the year. The Earth rotates about on a fixed plane that is tilted 23.5° with respect to its vertical axis around the sun. The Earth needs 23hrs 56mins to complete one true rotation, or one sidereal period, around the sun. The solar day, on the other hand, is the time needed for a point on earth pointing towards a particular point on the sun to complete one rotation and return to the same point. It is defined as the time taken for the sun to move from the zenith on one day to the zenith of the next day, or from noon today to noon tomorrow. The length of a solar day varies, and thus on the average is calculated to be 24hrs. In the course of the year, a solar day may differ to as much as 15mins. There are three reasons for this time difference.

  1. the earth"s motion around the Sun is not perfect circle but is eccentric;
  2. the Sun"s apparent motion is not parallel to the celestial equator;
  3. the precession of the Earth"s axis.

For simplicity, we averaged out that the Earth will complete one rotation every 24hrs (based on a solar day) and thus moves at a rate of 15° per hour (one full rotation is 360°). Because of this, the sun appears to move proportionately at a constant speed across the sky. The sun thus produces a daily solar arc, which is the apparent path of the sun"s motion across the sky. At different latitudes, the sun will travel across the sky at different angles each day.

The rotation of the earth about its axis also causes the day and night phenomenon. The length of the day and night depends on the time of the year and the latitude of the location. For places in the northern hemisphere, the shortest solar day occurs around December 21 (winter solstice) and the longest solar day occurs around June 21 (summer solstice). In theory, during the time of the equinox, the length of the day should be equal to the length of the night.

The average time the earth takes to move around the sun in approximately 365 days. This path that the earth takes to revolve around the sun is called the elliptical path.

Equinoxes & Solstices

Equinoxes happen when the ecliptic (sun"s apparent motion across the celestial sphere) and celestial equator intersect. When the sun is moving down from above the celestial equator, crosses it, then moves below it, that point of intersection between the two planes is when the Autumnal Equinox occurs. This usually happens around the 22nd of September. When the Sun moves up from below the celestial equator to above it, the point of intersection between the sun and the celestial equator is when Spring (Vernal) Equinox occurs. It usually happens around the 21st of March. During the equinoxes, all parts of the Earth experiences 12 hours of day and night and that is how equinox gets it name as equinox means equal night. At winter solstice (Dec), the North Pole is inclined directly away from the sun. 3 months later, the earth will reach the date point of the March equinox and that the sun"s declination will be 0°. 3 months later, the earth will reach the date point of the summer solstice. At this point it will be at declination -23.5°. This cycle will carry on, creating the seasons that we experience on earth.

The earth is tilted 23.5°, so is the ecliptic, with respect to the celestial equator, therefore the Sun maximum angular distance from the celestial equator is 23.5°. At the summer solstice which occurs around 21st of June, the North Pole is pointing towards the sun at an angle of 23.5° as shown in figure 1.3. Therefore the apparent declination of the sun is positive 23.5° with respect to the celestial equator. At the Winter solstice which occurs around 21st December, the North Pole is pointing away from the sun at an angle of 23.5°. Therefore the apparent declination of the sun is negative 23.5° with respect to the celestial equator.

Change of Seasons

Seasons are caused by the Earth axis which is tilted by 23.5° with respect to the ecliptic and due to the fact that the axis is always pointed to the same direction. When the northern axis is pointing to the direction of the Sun, it will be winter in the southern hemisphere and summer in the northern hemisphere. Northern hemisphere will experience summer because the Sun"s ray reached that part of the surface directly and more concentrated hence enabling that area to heat up more quickly. The southern hemisphere will receive the same amount of light ray at a more glancing angle, hence spreading out the light ray therefore is less concentrated and colder. The converse holds true when the Earth southern axis is pointing towards the Sun.

Sun apparent movement

From the heliocentric point of view, the Earth rotates and revolves around the sun in a counter clockwise direction. However, when we look at the Sun on earth, it appears to be moving in a clockwise direction. This phenomenon is known as the apparent motion of the sun.