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ASTA01H3 (100)
Lecture 1

# ASTA01H3 Lecture Notes - Lecture 1: Angular Diameter, Red, Rayleigh Scattering

Department
Astronomy
Course Code
ASTA01H3
Professor
Parandis Tajbakhsh
Lecture
1

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Precession
The Earth rotates around its axis of rotation daily.
The axis of rotation is tilted with respect to the vertical line to the plane of the orbit.
The line perpendicular to the orbit and the rotation axis of the Earth make an angle of 23.5
degrees.
The rotation axis of the Earth sweeps out a cone every 26,000 years.
This gradual wobble that alters the orientation of the rotation axis in space is called
precession.
This motion causes the north star and the position of all other stars to change.
Fun Fact: The angle between the Earth’s axis and the perpendicular to the plane of the
orbit is not constant and changes between 22.1° to 24.5° on a 41,000 year cycle.
The effect of precession causes the north star to change with time.
Celestial Coordinates
The celestial analogue of latitude is declination, elevation from celestial equator.
The celestial analogue of longitude is the right ascension.
The right ascension (RA) of a star is deﬁned as the angle it makes with the point of vernal
equinox as measured to the east, along the celestial equator.
The zero of right ascension is the point of vernal equinox.
The right ascension is measured in hours, minutes and seconds.
RA increases eastward.
The celestial sphere is divided into 24 hours. Thus each hour on the celestial sphere is
15°.
Sidereal vs. Synodic
Sidereal day is the amount of time that it takes a given star to make successive passes
across the local meridian. This takes 23 hours and 56 minutes.
During the sidereal day, the Earth rotates 360° with respect to remote stars.
The synodic day is deﬁned as the time period between two successive passages of the
sun from the local meridian.
The earth orbits more than 360 during a synodic day. A Synodic day is 4 minutes longer
than a sidereal day.
It takes moon 27.32 days to orbit the Earth once and return to the position it
started with respect to faraway stars (sidereal month).