Positional Astronomy Flashcards
Altitude
the angular height measured from the observer’s horizon
(tilt of the observing instrument)
the altitude of the pole is equal to the observer’s latitude
Azimuth
The angle measured from the Observer’s meridian, in the easterly direction.
(twist angle)
Celestial equator
The projection of the Earth’s equator onto the celestial sphere.
Circumpolar
Stars that remain above the horizon at all times
Conjuction
A planet lying in the same direction as the sun when viewed from the Earth
spherical cosine formula
cosa=cosbcosc+sinbsinccosA
Declination
The equivalent of latitude on the celestial sphere
Direct motion
Appears to move in the same direction as its true orbital motion when viewed from Earth.
Eg: sun’s ecliptic longitude increases uniformly throughout the course of the year.
Diurnal motion
The rotation of the Earth about its N-S axis accounts for diurnal motion (day and night)
Ecliptic
apparent path taken by the sun
titled at ε with respect to celestial equator
Elongation
ε is defined at the Earth as the angle made by the planet’s geocentric radius vector with the sun’s geocentric radius vector.
Equation of time
EqT=RAMS-RAS
where RA is right ascension
MS is mean sun
S is true sun
Equinox
the two points on the ecliptic where it intersects the celestial equator
Sun’s declination is zero
Geocentric
coordinate system with Earth at centre
eg: hour angle and declination
Great Circles
a circle that has the same radius as the sphere on which it is drawn
eg: equator
Hour Angle
equivalent to longitude on the celestial sphere
Inferior planet
planets whose orbits lie inside the Earth’s orbit
Julian Calendar
defined every 4th year to be a leap year to account for the remainder in the tropical year duration.
led to slippage in the annual recurrence of the vernal equinox
Latitude
circles on the surface of the Earth that are parallel to the equator
ranges from +90 to -90 (N to S)
equator is the zero of the latitude scale
Local siderial time
the hour angle of ♈︎ for the observer
LST=HA♈︎
Longitude
All meridians of longitude curves are great circles (NOT PARALLEL)
Arbitrary zero is the Greenwich Meridian
longitude=(LMST-UT)x15°
Maximum elongation
an inferior planet has maximum elongation when its geocentric radius vector is tangential to its orbit.
a superior planet has maximum elongation of 180°
Mean solar time
watches and clocks read mean solar time
(discrepancies due to Earth’s elliptical orbit and the ecliptic is not parallel to the celestial equator)
Mean sun
moves along celestial equator at same uniform average rate as the sun moves along the ecliptic.
M and S together at the equinoxes
Mean time
concept created using mean sun to have a uniform measure of time throughout the year
Nautical mile
the fraction of the Earth’s circumference subtending 1 minute of arc at the centre, as measured at the equator.
nautical mile is approximately 1.853km (using s=rθ)
North Celestial pole
The north pole of the Earth projected onto the celestial equator.
Obliquity of the ecliptic
the Earth’s rotational axis is tilted at 23.5 degrees with respect to its orbital axis. This angle is the obliquity of the ecliptic
Observer’s horizon
the projection of the observer’s terrestrial horizon.
the curve where the plane perpendicular to the line to the zenith
Observer’s meridian
the great circle arc that passes through the NCP and the observer’s zenith
Opposition
a superior planet that has an ecliptic longitude of 180 degrees from the sun
Sun-Earth-planet system is a straight line
inferior planets are never in opposition
Phase angle
Φ defined at the planet, by the Sun-Earth-Planet system
Quadrature
sets the value of planetary elongation to be 90 degrees
Retrograde Motion
At opposition, the planet appears to be moving in the direction opposite to its actual orbital motion when viewed from the Earth
Rise
non-circumpolar stars rise when it crosses from below the observer’s horizon into the visible sky
Set
non-circumpolar stars set when it crosses below the observer’s horizon
Siderial month
the moon completes its orbit around the Earth in 27.32 days relative to the distant stars
Siderial year
the time it takes Earth to complete one full orbit around the sun when viewed from distant stars
Spherical Sine formula
SinA/sina=SinB/sinb=SinC/sinc
Small circles
circles which do not have the same radius as the sphere
Solar time
The non-uniform time recorded by the sundial
Solstice
The points where the sun has maximum/minimum declination (21st June and 21st December)
Superior planet
planets whose orbit lies outside the Earth’s orbit
Synodic period
Since the Earth is also in orbit around the sun, the period for the moon to complete its orbit is 2.2 days longer than the sidereal.
Topocentric
observer at the centre of the universe
Transit
when the star crosses the observer’s meridian
Tropical year
the period from 1 vernal equinox to the next, comprising 365.2422 mean solar days
Tropics
the lines of latitude +/-ε define the tropics of cancer and capricorn
Zenith
the point directly over-head of the observer.
Zenith distance
the distance to the star as measured from the zenith down towards the horizon
Zenith distance=90 degrees - a
Zodiac
13 constellations make up the zodiac
sun appears to move steadily eastwards around the ecliptic, passing through each of the constellations.
