Introduction to Astronomy AST0111-3 (Astronomía) Semester 2014B Prof. Thomas H. Puzia
Theme Our Sky 1. Celestial Sphere 2. Diurnal Movement 3. Annual Movement 4. Lunar Movement 5. The Seasons 6. Eclipses
Celestial Sphere When observing the sky, we see 1000s of stars The Sun moves daily Day and night Stars remain fixed with respect to each other The Sun also moves annually on sky The Moon has phases and moves on sky Eclipses and occultations
Constellations are official regions on the night sky, although we often identify via the stars within them. Stars can be very far from each other, although they appear close on the sky. The background stars appear to be fixed with respect to one another, implying they must be very distant. Constellations appear today as they did 2000 years ago Positions on the sky = angular distance measurements. Celestial Sphere
Movements of the Earth Rotate on its axis Orbit around the Sun Precession of its axis Nutation of its axis Revolution around barycenter Chaotic movements Movement of the Sun Rotation of our Galaxy Movement of our Galaxy
Movements of the Earth
Diurnal (Daily) Movement Why do stars rise and set?
Diurnal Movement Local sky reference coordinates: horizon, zenith.
Diurnal Movement The celestial sphere rotates around the extension of the Earth s polar axis Finding the Celestial South Pole:
Diurnal Movement We also define absolute reference coordinates: - Celestial N and S pole, - Celestial Equator. Extend Earth pole/equator out into the sky. 1 complete turn of sky = 1 sidereal day = 23h 56m Note: N is up by convention If we want to observe a star from Earth for 6 hrs, how can we do that?
The sidereal day is shorter than the solar day. How does Earth rotate on the path of its orbit? A. Earth rotates in the same sense as its orbital motion B. Earth rotates in the opposite sense as its orbital motion C. That depends on the position of the observer on Earth
Diurnal Movement How do we observe diurnal movement at: " The Equator? " The Poles?
Annual (Yearly) Movement
Annual Movement 1 revolution = 1 sidereal year = 365.245 d. Plane of the Earth s equator: Celestial Equator Plane of the Earth s orbit: Ecliptic
Annual Movement The ecliptic marks the path of the Sun on the celestial sphere
Earth s orbit only deviates from circle by 1.5% The inclination of the ecliptic with respect to the Earth s equator is 23.5 o This means the Earth s axis of rotation is inclined with respect to the axis of the ecliptic. The seasons of the year are caused by the inclination of the axis of rotation of the Earth (which dictates length of day vs. night).
Annual Movement Equinoxes, There are two points marking the intersection between the planes of the ecliptic and the Earth s equator. Day and Night are both 12 hrs on: 1. Vernal Equinox of Mar 21st 2. Spring Equinox of Sep 21st Solstice, 1. of Summer, southern most point of the ecliptic, on Dec 21, the longest day of the year. 2. of Winter, northern most point of the ecliptic, Jun 21, longest night of the year.
N Jun21 Dec21 S The highest point of the Sun above a point on the Earth s surface varies during the year. The resulting high and low variations of light mark the passage of the seasons. NOTE: not because the Sun is closer or farther away from the Earth. Annual Movement: The Seasons Annalema
Annual Movement As a consequence of the Earth s orbit around the Sun, the Sun appears to move ~1 degree on the sky per day with respect to the fixed background stars. This movement produces a difference between the sidereal day (23h 56m) and the solar day (24h). Calculation: 360 degrees in 24 hours 1 degree in 4 minutes
Annual Movement: The Zodiac In its apparent movement on the sky, the Sun travels through distinct constellations over the length of the year. These are known as the Zodiac constellations. There are nominally 12, one for each month.
The four seasons (spring, summer, fall, winter) are caused by: A. The tilt of the Earth s axis of rotation B. Variations in distance between the Sun and the Earth C. The tides and the oceans D. The gravitational pull of the black hole in the center of our Galaxy E. The variable speed of Earth s orbit around the Sun