Winter Sky – Facing North

Celestial Motions The rotation of the Earth about its spin axis once every 24 hours causes diurnal effects: day and night and the “rising” and “setting” of celestial objects. The revolution of the Earth about the sun once every 365.2422… days produces annual effects: the sun appearing to move with respect to the stars along a path in the sky called the ecliptic. The twelve constellations lying along the ecliptic comprise the zodiac. The apparent motions of celestial objects on the sky are the combined result of diurnal and annual motions and, in the case of the planets their own orbital motions around the sun.

The Celestial Sphere north celestial pole Earth’s spin axis north pole equator celestial equator south pole celestial sphere south celestial pole

the “ecliptic plane” (plane of the Earth’s orbit around the sun) Tilt of Earth’s Spin Axis 23.5o tilt The Earth’s spin axis is tilted by 23.5 degrees off vertical with respect to the “ecliptic plane” (plane of the Earth’s orbit around the sun) The spin axis remains essentially parallel to itself during the course of the year

Summer Solstice – 21 June tropic of Cancer arctic circle equator sunlight equator antarctic circle sunlight

Winter Solstice – 21 December tropic of Cancer arctic circle sunlight equator sunlight antarctic circle tropic of Capricorn

Vernal Equinox – 21 March arctic circle tropic of Cancer equator sunlight tropic of Cancer equator tropic of Capricorn sunlight antarctic circle

Autumnal Equinox – 21 September arctic circle sunlight tropic of Cancer equator tropic of Capricorn sunlight antarctic circle

The Culminating Sun The sun culminates in the zenith (i.e. straight overhead) at noon for observers located on the tropic of Cancer (latitude = 23.5o N) on the day of the summer solstice. The sun culminates in the zenith at noon for observers located on the tropic of Capricorn (latitude = 23.5o S) on the day of the winter solstice. The sun culminates in the zenith (i.e. straight overhead) at noon for observers located on the equator (latitude = 0o) on the days of the equinoxes.

Other Seasonal Extremes The sun never rises for observers north of the arctic circle on the day of the winter solstice The sun never sets for observers north of the arctic circle on the day of the summer solstice The above conditions are reversed for the antarctic circle. The sun moves 360o around the horizon for observers located at the north and south poles on the days of the equinoxes.

Local Position

Tilt of Earth’s Spin Axis W N S E winter solstice equinox summer solstice The sun rises on the east point and sets on the west point on the days of the equinoxes, giving equal periods of “day” and “night”. The sun is in the sky for the longest duration on the summer solstice and illuminates the northern hemisphere most directly.

Temperature Effect Summer days are longer and the sun is more intense (due to the more direct illumination angle). Thus summer is hotter than winter. There is a lag of the seasons when comparing the dates of the solstices with the actual extremes in temperature because it takes time to heat up the oceans and atmosphere at the onset of summer and to cool them off at the onset of winter. If the Earth’s spin axis were not tilted by some angle, we would have no seasons.

Motion of Sun, Moon and other Planets All major bodies in the Solar System move around ecliptic Slow drift (from W to E) against the background of stars Skyglobe demo 7 visible “planets” incl. the sun and moon Planet = “wanderer” Days of the week named after the planets

Why do all planets move in the same plane? Reason: Formation process of the Solar System Condenses from a rotating cloud of gas and dust Conservation of angular momentum flattens it Dust helps cool the nebula and acts as seeds for the clumping of matter

Formation of Planets Orbiting dust – planitesimals Planitesimals collide Different elements form in different regions due to temperature Asteroids Remaining gas

The Zodiac 12 h RA - Libra the Scales 0 h RA - Aries the Ram The Zodiac is: The ecliptic is divided into 12 segments Each segment subtends 30º or 2 h of RA Your zodiac “sign” is the constellation that the sun was in when you were born 12 h RA - Libra the Scales 30º 0 h RA - Aries the Ram *Note: Due to precession of Earth’s axis, the position of the sun in the zodiac constellations is actually shifting Image taken from http://www.astrologyclub.org/articles/ecliptic/ecliptic2.gif

Precession

2. Moon’s orbital plane is tilted by 5o from our equator Precession of Earth’s Spin Axis 5. Spin axis now points to Polaris. 13,000 years from now, Vega will be our “pole star” 4. Earth responds to this pull by slowly “precessing” its spin axis around a circle in the sky once every 26,000 years 23.5o 2. Moon’s orbital plane is tilted by 5o from our equator 3. Moon’s gravitational pull on Earth attempts to pull bulge into lunar orbital plane 1. The rotation of the Earth distorts it into an “oblate” spheroid flattened at the poles

http://www. upscale. utoronto http://www.upscale.utoronto.ca/PVB/Harrison/Flash/ClassMechanics/Precession/Precession.html