Astronomy 101: Understanding the Celestial Sphere and Annual Movement, Exams of Astronomy

Download Astronomy 101: Understanding the Celestial Sphere and Annual Movement and more Exams Astronomy in PDF only on Docsity!

Introduction to

Astronomy

AST0111-3 (Astronomía)

Semester 2014 B 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 Sphere

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.

! (^) 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?

Local sky reference coordinates: horizon, zenith.

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

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

! (^) Earth’s orbit only deviates from circle by 1.5% ! (^) The inclination of the ecliptic with respect to the Earth’s equator is 23. 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).

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,
3. 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

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.