intro chapter 2a coordinatesystems

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The Earth

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The Celestial Sphere The celestial sphere Zenith Nadir meridian

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The Celestial Sphere The celestial sphere Zenith Nadir meridian

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The Horizontal System W E N S Zenith meridian

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The altitude-azimuth system Although simple, the altitude-azimuth system is difficult to use: depends on location of the observer on Earth since the Earth rotates, stars appear to move constantly across the sky coordinates change constantly the stars rise 24h/365 ~ 4 minutes earlier on each successive night As coordinates change from day to day it seems wise to adopt a more convenient coordinate system within a year we see the whole celestial sky the equatorial system

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N S W E Zenith

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David Malin Anglo-Australian Observatory

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At the North Pole S S W E North Celestial Pole celestial equator = horizon = Zenith Stars with are not visible

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N S W E Zenith North Celestial Pole celestial equator At the Equator All the stars are visible

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The Motion of the Sun The Earth moves around the Sun in an orbital plane. The orbital plane‘s projection onto the celestial sky defines the Zodiac 24h sidereal time corresponds to 24h+24/365h (Unsöld, Baschek)

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north celestial pole equator vernal equinox Mar 20 day = night autumnal equinox Sept. 23 zenith

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June, 21 Summer solstice maximum altitude: Sun in zenith zenith minimum altitude: North polar day

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Dec 21 Winter solstice maximum altitude: North polar night zenith

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The Precession of the Sun 23.5° ecliptic = orbital plane of the planets The axis of the Earth describes a cone once in 25725 years.