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Lecture 5

ASTA01H3 Lecture Notes - Lecture 5: Celestial Equator, Tropical Year, Right Ascension

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Kristen Menou

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- Stellar coordinates provides a coordinate grid on the sky that tells the position of any star, just as
longitude and latitude tells us the position of any point on Earth
- “Latitude” is called declination and “Longitude” is called right ascension
- Declination is expressed in degrees, arc minutes, and arc seconds north (+) or south (-) of the celestial
- Capella, the brightest star in the constellation Auriga, has a declination of +46 degrees 0’, and is
therefore, about halfway between the celestial equator (dec = 0 degrees) and the north celestial pole
(dec = 90 degrees)
- Right ascension is expressed not in degrees but in hours (h), minutes (m), and seconds (s) of time, from
0 to 24 hours
- Since the Earth completes one turn in 24 hours, the celestial sphere appears to take 24 hours to
complete one turn around Earth
- Another way of determining the length of a day is to measure the time it takes for any star to make
successive passes across the local meridian which we call a sidereal day
- A sidereal day is about 23 hours 56 min, shorter than a solar day by about 4min
- This is due to during the solar day, the Earth has travelled along its orbit around the Sun, and the Earth
needs a little more time to rotate before the Sun crosses the meridian
- Timekeeping involving months comes from the lunar phase’s cycle, which is about 29.5 solar days
- This roughly corresponds to the average month length, known as a synodic month
- If we use the stars to measure the length of the lunar cycle, a sidereal month, the time turns out to be
27.3 days
- The length of a year is clearly related to the time it takes the Earth to complete one full orbit around
the Sun which is 365.25 days
- A sidereal year is the time taken for a complete orbit relative to the stars, whereas the time between
successive spring equinoxes is called a tropical year
- A sidereal year is longer than a tropical year by about 20min, the difference due to the precession of
the Earths rotation
- We use an aspect of solar time: The apparent solar time is determined by the Sun’s position in the sky
relative to our local meridian
-When the Sun is right on the meridian, it is noon
- Before the Sun gets to the meridian we say that it is ante meridian (AM)
- When the Sun has passed the meridian we say that it is post meridian (PM)
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