The Sun in the Sky And how it changes in the course of the year But firstlast time we saw that you can determine your latitude from the altitude angle of Polaris (the north
star) There is a better (and more accurate) way to do it. zenith Meridian plane To polaris
north lat Surface of Earth south zenith
Meridian plane To polaris A2=lat+90 north lat
Surface of Earth south zenith Meridian plane To polaris
A2=lat+90 north lat A3 Surface of Earth A3=altitude angle of celestial equator=180-(A2) A3=90-lat
south zenith Meridian plane To polaris dec
A2=lat+90 north lat Surface of Earth A3 south
zenith Meridian plane To polaris alt dec A2=lat+90 north
lat A3 south Surface of Earth Alt=altitude angle of star at transit=A3+dec=90-lat+dec
The basis of navigation at sea (at least for latitude) Your latitude can be precisely measured by the altitude angle (alt) at which a star with declination (dec) transits (crosses the meridian) Alt=90-lat+dec So, lat=90+dec-alt Lets return to a concept from last time
How do we understand these changes during the year? Weve done this, so what did it do for us? Method 1: introduce a second coordinate system for use on the sky
Method 2: understand the physics of the solar system (later) New coordinate system is like defining your location on Earth (what are the coordinates for locating a position on Earth?) Measuring the position of the Sun against the background stars
The path of the Sun through the stars An important astronomical fact: During the course of the year, the Sun moves against the background stars, just like the planets Find the ecliptic on the SC1 chart Another great circle on the skythe ecliptic
Question: why is it 23.5 degrees? What does that sound like? All annual variations (or most of them) can be explained as due to the changing right ascension and
declination of the Sun The fact that the Sun is lower in the sky in the winter than the summer The phenomena of the solstices and equinoxes The change in the length of the day Its colder in the winter! Important terms and concepts in the equatorial coordinate system
Celestial equator North and south celestial pole Right ascension (coordinate like longitude, only units are hours, minutes) Declination (coordinate like latitude) Ecliptic Vernal equinox (sometimes called the first point of Aries) Lets use the SC1 chart to learn
something about astronomy Plot up the positions of the planets (in pencil!) Current positions of the major planets
Mercury: RA=19h11m, Dec=-22d Venus: RA=21h13m, Dec=-17.5d Mars: RA=8h52m, Dec=22.3d Jupiter: RA=22h20m, Dec=-11.3d Saturn: RA=12h19m, Dec=0.5d
Uranus: RA=23h37m, Dec=-3.3d Neptune: RA=21h49m, Dec=-13.6d Where are they? Are there any systematic trends? Before going to an explanation (what does it all mean?) A description of the orbit of the Moon. Any opinions on where you can find the Moon in the sky?
The reason for phases of the Moon Phases of the moon and position of the Sun Phases of the moon
