Celestial coordinates
Just as a place on Earth is fixed by latitude and longitude, a body in the sky is fixed by two angles. In the horizontal system these are altitude h (from the horizon) and azimuth A (in this course, following the school-astronomy rule, measured along the horizon from the south point westward, 0° to 360°; in geodesy and navigation it is counted from north through east instead); these depend on the observer’s place and time and change during the day. In the equatorial system we give declination δ (from the celestial equator, –90° to +90°) and right ascension α (from the vernal equinox eastward, 0 to 24 hours, 1 h = 15°, 1 min = 15′); these turn with the stars and suit catalogues because they do not change in the daily motion. The ecliptic system uses ecliptic longitude λ and latitude β – handy for Solar System bodies. The hour angle t is measured along the celestial equator westward from the southern part of the meridian and links the equatorial system to local sidereal time.
Together with an adult, give “horizontal” coordinates to objects seen from the classroom: with a protractor measure the altitude and azimuth of a treetop seen through a window. Do not look at the Sun.