Lecture 4

Class AnnouncementsMonday, August 28: 

Class Announcements Monday, August 28 Labs start tonight, 8pm -10 pm Homework 1 due today at end of lecture Homework 2 on course website Observing/ Clear Sky patrol Starts tomorrow night. Observing list on website late today East elevator, 7th floor VAN Door next to elevator

Slide2: 

Ecliptic: the path of the Sun (and planets) in the sky complicated by the fact that Earth is tilted on its own axis by 23.5 degrees Note: Constellations of the zodiac lie on the path of the ecliptic, since they are ’behind’ the Sun’s path.

Slide3: 

Observer at North Pole:Where is Sun’s position on Summer Solstice (June 21) ? North Pole: Sun is 23.5 above the horizon all day on June 21

Observer at Equator (e.g Quito): Where is Sun on March 21 (Vernal equinox)?: 

Observer at Equator (e.g Quito): Where is Sun on March 21 (Vernal equinox)? Zenith at noon Mach 21 Observer at equator on March 21 (or Sept 21): Sun rises in E, at zenith at noon, sets in W)

Ecliptic, Celestial Planes and the Path of the Sun: 

Ecliptic, Celestial Planes and the Path of the Sun

Path of Sun from March 21 to Sept 21: 

Path of Sun from March 21 to Sept 21 Sun today March 21 June 21 Sept 21

Slide7: 

As the Earth moves in its orbit, The path of the Sun (and planets) changes because of the tilt of the rotation axis Note: Since rotation axis is fixed with respect to the stars, star paths do NOT change!

Slide8: 

Northern Hemisphere Winter Northern Hemisphere Summer

Slide9: 

What dates correspond to each figure? 1 2 3 4

Slide10: 

Cause of seasons

Azimuth of sunrise changes with season: 

Azimuth of sunrise changes with season East Northeast Southeast Dawn

Stonehenge: Ancient astronomical site aligned to solar azimuth: 

Stonehenge: Ancient astronomical site aligned to solar azimuth

Slide14: 

N. Summer: Northern Hemisphere tilted toward Sun N. Winter: Southern Hemisphere tilted toward Sun Seasons: Summary Due to the Earth’s 23.5º tilt on its axis 4 important dates: 1. Summer solstice (June 21)– sun most northerly on ecliptic 2. Autumnal equinox (Sep 21) – sun crosses celestial equator 3. Winter solstice (Dec 21) – sun most southerly on ecliptic 4. Vernal (spring) equinox (Mar 21) – sun crosses c. equator Myth: The Earth is closer to Sun in summer Wrong! The sun’s rays reach (N. hemisphere) of the Earth more DIRECTLY in (Northern H.) summer, less DIRECTLY in winter (doesn’t have to do with sun-earth distance)

Slide15: 

Coordinates in the sky: Horizon System Zenith: point directly overhead at any time Horizon: the lowest point you can see (the ground)

Slide16: 

Celestial Sphere: Extension of the Earth’s Coordinate System celestial sphere N/S celestial poles celestial equator Like a salad bowl over your head!

Slide17: 

Apparent westward motion of sky Due to Earth’s rotation about its axis every 24h in an EASTWARD direction To us, it appears as if objects in the sky are moving WESTWARD— Sun rises in the E, Sun sets in W

Slide18: 

The Night Sky at the North Pole Polaris (N. Celestial Pole) fixed overhead Time-lapse picture from the North Pole looking straight up (zenith)

Slide19: 

The Night Sky at the Equator North & South Celestial Poles are on the horizon North Celestial Pole (North Star)

Slide20: 

Most of us are in between these extremes we see fixed Polaris (N. Celestial Pole star) over the night, stars move WESTWARD about it E W Looking North

Transit: Crossing the observer’s prime meridian: 

Transit: Crossing the observer’s prime meridian All celestial bodies rise in the east, set in the west A line running from N to S crossing the zenith is the prime meridian Objects transit when crossing this imaginary line Transiting objects are always due south and are highest in the sky (highest elevation) Prime meridian line Transit

Prime Meridian: 

Prime Meridian

Earth’s rotation and the apparent motion of stars: 

Earth’s rotation and the apparent motion of stars All celestial bodies (stars, planets, sun, moon) appear to move across the sky east to west This is caused by the Earth rotating (west to east) on its own axis, like a spinning top. The Earth’s rotational period is 23h 56m 4s. (NOT 24h!) This is called the sidereal period. Hence, all stars are at the same position in the sky every successive sidereal period. This implies that all stars rise appx. 4 minutes earlier each night. Since 4 min x 365 days ~ 24 hr, the night sky patterns repeat annually for a given time of night

Daily Motion of Stars, Solar system objects on the Sky: 

Daily Motion of Stars, Solar system objects on the Sky Stars move along the same westward track every night (ignores precession, discussed later in lecture) Stars rise, transit, and set 4 min earlier every night. Solar system objects also move east to west each night, but their tracks are more complicated (because apparent motion is affected by Earth’s orbital motion around Sun) The Sun and all planets (except Pluto) move along the line of the ecliptic plane, described in next slide. Planets generally move eastward with respect to the stellar background, except during retrograde motion (Chap 3) The Moon’s motion is the most complicated: we will discuss this in chapter 2.

Night sky: The Movie Iowa City looking south: 

Night sky: The Movie Iowa City looking south East West

Night sky at 24 hour intervals (Solar period): 

Night sky at 24 hour intervals (Solar period)

Slide27: 

Night sky at 23h 56m 4s intervals (Sidereal period)

Motion of the Sun: Solar vs. Sidereal period: 

Motion of the Sun: Solar vs. Sidereal period Since the Earth moves in its orbit each day, the motion of (nearby) solar system objects is slightly different from stars. The Earth needs to rotate an additional 1 degree daily for the Sun to repeat its position (see diagram at right). This takes an additional 4 min This means that the Sun’s position repeats every 24 hrs. This is called the solar period.