Introduction to orientation on the sky
Andreja GombocSchool of ScienceUniversity of Nova Gorica
celestial sphere
celestial sphere - view of an observer on Earth
description of a position on a sphere - example: Earth
• north and south Earth pole • Earth’s equator • Greenwich meridian • longitude λ • latitude ϕ
• zenith and nadir • horizon • celestial meridian (to South) • azimuth A • altitude h (or zenith distance z)
description of a position on celestial sphere - horizontal system
animation: http://astro.unl.edu/naap/motion2/animations/ce_hc.html
observing place + Earth’s rotation
description of a position on celestial sphere - equatorial system 1
• north and south celestial pole • celestial equator • celestial meridian (to South) • hour angle H • declination δ
• hour angle H increases linearly with time
apparent daily motion of stars
animation: http://astro.unl.edu/naap/motion2/animations/ce_hc.html
apparent daily motion of stars as seen from different observing sites on Earth
description of a position on celestial sphere - equatorial system 2
• north and south celestial pole • celestial equator • vernal equinox - gamma point • right ascension alfa • declination δ
right ascension and declination of stars do not depend on observing site and time*
* if we do not take into account proper motion of stars
upper and lower culmination
setting of a star
yearly changing of the night sky
apparent motion of the Sun
• daily - similar as stars - apparently circles aroung the north celestial pole (due to Earth’s rotation): rises, culminates, sets…
• yearly - different, due to motion of Earth around the Sun
motion of Earth around the Sun
• seasons
animation of light-rays direction: http://astro.unl.edu/naap/motion1/animations/seasons_ecliptic.html
ecliptic - constellations of the Zodiac
right ascension and declination of the Sun change and repeat after one year
at spring equinox the Sun is in vernal equinox - gamma point
Sun apparently moves along the ecliptic
altitude of the Sun and length of a day
animation: http://astro.unl.edu/naap/motion3/animations/sunmotions.html
altitude of the Sun, day length, direction and length of a shadow
• animation University of Nebraska-Lincoln:
try for yourself and change: • day in the year • observing site • … • observe Sun’s position on the sky at 12:00 local time
animation: http://astro.unl.edu/naap/motion3/animations/sunmotions.html
apparent motion of the Moon
Moon phases
relation between sidereal (P*) and synodic (PS) period
• P*Moon=27.3 days, PSMoon=29.5 days
motion of planets
• close to the ecliptic plane
retrograde motion