Introduction to Astronomy !AST0111-3 (Astronomía)

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Semester 2014B

Prof. Thomas H. Puzia

1. Celestial Sphere

2. Diurnal Movement

3. Annual Movement

4. Lunar Movement

5. The Seasons

6. Eclipses

Theme Our Sky

Celestial SphereWhen 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

Celestial SphereConstellations 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.

! 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

Movements of the Earth

Diurnal (Daily) Movement

Why do stars rise and set?

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?

Diurnal Movement

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

How do we observe diurnal movement at: " The Equator? " The Poles?

Diurnal Movement

Annual (Yearly) 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

Annual Movement

! Earth’s orbit only deviates from circle by 1.5% ! The inclination of the ecliptic with respect to the Earth’s

equator is 23.5o

! 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).

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.

Annual Movement

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.Jun21 Dec21

N S

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