Unit 1 Physics Detailed Study 3.1 Chapter 10: Astronomy

Section 10.2 The Sun, the Moon and the planets The Sun It is a relatively easy job to plot the Sun's position on the celestial sphere and in that way determine its path through the stars 10.2 Start

Section 10.2 The Sun, the Moon and the planets The Earth's axis remains on the same tilt throughout the year causing the seasons. In December, when the South Pole is tilted towards the Sun, the southern hemisphere has summer. The date when the Sun is highest is known as the summer solstice (~Dec 21) The winter solstice (~June 21) is the date when the Sun is lowest in the southern hemisphere sky. In the northern hemisphere the solstices are the other way around. At the March and September equinoxes the tilt is such that the boundary between day and night runs right through the poles. This means that everywhere on the Earth has 12 hours of daylight and 12 hours of night-time; 'equinox' (equal day and night). After the March equinox the South Pole will move into 24-hour night and the North Pole into 24-hour day. Only the equator will experience equal day and night all year round.

Section 10.2 The Sun, the Moon and the planets NCP – North Celestial Pole

Section 10.2 The Sun, the Moon and the planets The Earth's axis remains on the same tilt, relative to the plane of its orbit, throughout the yearly revolution. It is the tilt of the axis that gives rise to the seasons. In December, when the South Pole is tilted towards the Sun, the southern hemisphere NCP – North Celestial Pole

Section 10.2 The Sun, the Moon and the planets The Sun moves along the ecliptic The Sun ‘describes’ a circular path against the stars once a year. Due to the tilt of the Earth's axis, the path the Sun follows through the stars is also on a tilt relative to the celestial equator. The path is referred to as the ecliptic The relationship is shown in Fig 10.21. The two points where the Sun crosses from one hemisphere to the other are known as the vernal (March) and autumnal (September) equinoxes. Remember that the Sun's daily path through our sky is due to the Earth's daily rotation, not our motion around the Sun. The Sun only moves about 1° through the celestial sphere each day.

Section 10.2 The Sun, the Moon and the planets The Sun moves along the ecliptic The band of constellations through which the Sun passes each year is referred to as the Zodiac. These are the constellations that astrologers refer to and so have familiar names such as Pisces, Aquarius, and so on (see Figure 10.22). Your 'astrological sign' is supposed to be the constellation the Sun is in at your birth

Section 10.2 The Sun, the Moon and the planets The moon – its phases and eclipses The phases of the moon are related to its position with respect to the Sun and Earth. A sidereal month (27 1/3 days) is the time for the moon to revolve around once (rel. to bg stars) A syndonic month (29 ½ days) is the time between two new moons. Eclipses occur when the Moon and the Earth are in a direct line.

Section 10.2 The Sun, the Moon and the planets The wandering planets The stars follow exactly the same circular paths every day and every year. The Sun and Moon move easterly through the celestial sphere along the ecliptic at fairly constant rates, repeating their paths every year and every month. By comparison, the planets are quite untidy in their wanderings through the celestial sphere 'planet' means 'wanderer' in ancient Greek. Although they stay reasonably close to the ecliptic, their motion along it is quite variable—they speed up and slow down, sometimes even going 'backwards', or in a retrograde motion.