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Astro midterm part 4
| Question | Answer |
|---|---|
| Sidereal | means "of or pertaining to the stars" The earth rotates once (360 degrees) every 23H 56m 4s relative to the stars (siderieal day) |
| What is a Solar Day? | A Solar Day is 24 hours measured from noon to noon. |
| How much does the Earth rotate in a Solar Day? | The Earth rotates slightly over 360 degrees in a Solar Day. |
| How much earlier do stars rise each night? | Stars rise 4 minutes earlier every night. |
| What is another name for a Solar Day? | A Solar Day is also known as a Synodic day. |
| What is sidereal time relative to? | The stars |
| How often does each star rise at the same sidereal time? | Every night |
| Does the sidereal time for stars change with the time of year? | No, it does not change with the time of year. |
| Does sidereal time vary with different longitudes? | Yes, it is different at different longitudes. |
| How much shorter is a sidereal day compared to a solar day? | 3 minutes and 56.56 seconds |
| What type of periods are the rotational periods of planets considered? | Sidereal |
| What are the lengths of days based on the position of the sun called? | Solar or synodic |
| Local Sidereal time | is the HA of the vernal equinox. or LST=HA(of object)+RA(of same object). OR LST=RA of an object at the meridian |
| Sidereal year | 365.2564 days (the time it takes the sun to return to the same position in the sky with respect to the stars) 360 degrees |
| Tropical year | 365.2422 days (vernal equinox to vernal equinox-smaller than sidereal due to precession) basis for calendars |
| What is precession in astronomy? | Precession is the gradual change in the orientation of the Earth's rotational axis. |
| What causes the Earth's pole to precess? | The gravitational forces from the Sun, Moon, and planets pull on the Earth's equator, creating a torque. |
| What is the period of Earth's precession? | The period of Earth's precession is approximately 26,000 years. |
| How does precession affect celestial coordinates? | The right ascension (RA) and declination (dec) of a star slowly change as the Earth precesses. |
| What celestial features precess along with the Earth's pole? | The North and South Celestial Poles, the celestial equator, and all lines of right ascension and declination precess. |
| Greenwich Mean Time or Universal Time | AKA Universal Coordinated Time (UTC). We live in times zones that are defined relative to this. To keep UTC in step with sidereal time we account for precession, nutation and slowing of earths rotation so we add leap seconds to the clock when necessary |
| Julian Calendar | Includes the leap year, 365 days. Didn't keep perfect time. By 1582, the julian calendar was 10 days out of sync with sidereal time |
| Gregorian calendar | We use this today. every 4 year is a leap year except for the years divisible by 100, of these, only the years divisible by 400 are leap years. Is still not perfect. We will lose one day every 3300 years |
| Parallax | is the apparent shift in position of a foreground object when viewed against more distant, background objects |
| To get the most accurate measurement you want the longest | baseline, so to measure parallax in astronomical objects you would observe the object at 2 different times, separated by 6 months. But this measurement is twice the parallactic angle (p) taken over twice the baseline (d) |
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