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