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Lecture 1

ASTA01H3 Lecture Notes - Lecture 1: Orbital Period, Apsis, Tidal Locking

Course Code
Parandis Tajbakhsh

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Local Sky
Zenith: The point exactly above your head.
Horizon: All points 90 degrees from Zenith.
Local Meridian: The circle that passes through North, South and Zenith
An object's altitude (above horizon) and direction (along horizon) specify its location in
your local sky.
Altitude: the elevation of a star above the local horizon measured in degrees.
Motion of Stars in the Sky
A star rises when it comes above the horizon and sets when it goes below the horizon.
The larger the declination, the farther from the point of west the star sets.
If the declination of a star is larger than 90 - latitude, then that star never sets and always
remain above the horizon.
Appearance of Local Sky
Circumpolar constellations are those which always stay above the horizon and never set.
The declination of a circumpolar star is equal or larger than = 90 - Φ
From higher latitudes you see more circumpolar constellations since (90 - latitude) is a
smaller number and thus more stars qualify as circumpolar.
From Mid latitude, some stars are circumpolar, some rise and set and some never rise
above the horizon.
The Ecliptic
The Sun appears to orbit the Earth once a year on a path called the Ecliptic.
The direction of this motion is from west to east (counter clockwise), contrary to the daily
motion which is from east to west.
There are 360° in a circle but 365 days in a year so the Sun moves on its annual path by
less than 1° every day.
Each day the sun moves about a degree eastward in addition to its daily east-west
The points where the Ecliptic intersects the celestial equator are known as the Vernal
Equinox and Autumnal Equinox.
When the Sun is on the point of Vernal Equinox (roughly March 20th) or Autumnal Equinox
(Roughly September 22nd) it has a 0° declination.
At the equinoxes, the sun rises exactly in the East and sets exactly in the West.
The length of the day and night is equal, each being 12 hours, thus the term equinox.
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