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Lecture 2

ASTR 1 Lecture Notes - Lecture 2: Celestial Equator, Celestial Coordinate System, Big Bang

2 pages60 viewsSummer 2018

Astronomy and Astrophysics
Course Code
Jean Brodie

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Where did the milky way and other galaxies come from?
- Inflation
- Big bang - A rapid and dramatic phase of expansion shortly after the big bang
blows up these quantum fluctuations and “freezes” them into the fabric of the
universe. These are the seeds of the cosmic structure.
Chapter 2 (2.1 - 2.2): The View From Earth
- The celestial sphere
- Motion of the Earth
- The earth spins on its axis (once per day)
- Everything in the sky appears to rise and set once per rotation (i.e.
once per day)
- Star trails show this motion
- Everything (beyond the solar system) appears fixed on the sky
- This is because they are so far away that moment
becomes negligible anyway. Our motion does not make
them appear to move.
- Constellations are fixed arrangements of stars that are not
necessarily physically related
- We can define a coordinate system as on Earth:
- Absolute coordinates (unchanging):
- North Pole
- South Pole
- Celestial equator
- An aside - The Earth’s precesses (like a top) once per 26,000
years, the celestial north pole moves in a circle over time
- We can define a coordinate system as on Earth:
- Absolute coordinates
- Declination (~latitude)
- Is measured -90 to 0 to 90 (pole - equator -
- Right ascension (~longitude)
- Is measured 0 to 360
- The earth orbits the sun (axis tilted with respect to orbit)
- Half the sky is overhead during the day
- The earth orbits the sun once per year
- This makes a convenient way to define RA=0
- Ecliptic path= apparent path of the sun through the stars
- Ecliptic plane= Plane of the earth’s motion (or the sun’s
apparent motion)
- Let the Sun mark the location on RA=0 deg on the vernal equinox
- Zodiac= constellations on the ecliptic plane
- Defined in 100AD… constellations/dates are slightly off now. This
is because of precession.
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