Positional Astronomy

Question 1

Altitude

Answer 1

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

Question 2

Azimuth

Answer 2

The angle measured from the Observer’s meridian, in the easterly direction.
(twist angle)

Question 3

Celestial equator

Answer 3

The projection of the Earth’s equator onto the celestial sphere.

Question 4

Circumpolar

Answer 4

Stars that remain above the horizon at all times

Question 5

Conjuction

Answer 5

A planet lying in the same direction as the sun when viewed from the Earth

Question 6

spherical cosine formula

Answer 6

cosa=cosbcosc+sinbsinccosA

Question 7

Declination

Answer 7

The equivalent of latitude on the celestial sphere

Question 8

Direct motion

Answer 8

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.

Question 9

Diurnal motion

Answer 9

The rotation of the Earth about its N-S axis accounts for diurnal motion (day and night)

Question 10

Ecliptic

Answer 10

apparent path taken by the sun

titled at ε with respect to celestial equator

Question 11

Elongation

Answer 11

ε is defined at the Earth as the angle made by the planet’s geocentric radius vector with the sun’s geocentric radius vector.

Question 12

Equation of time

Answer 12

EqT=RAMS-RAS

where RA is right ascension
MS is mean sun
S is true sun

Question 13

Equinox

Answer 13

the two points on the ecliptic where it intersects the celestial equator
Sun’s declination is zero

Question 14

Geocentric

Answer 14

coordinate system with Earth at centre
eg: hour angle and declination

Question 15

Great Circles

Answer 15

a circle that has the same radius as the sphere on which it is drawn
eg: equator

Question 16

Hour Angle

Answer 16

equivalent to longitude on the celestial sphere

Question 17

Inferior planet

Answer 17

planets whose orbits lie inside the Earth’s orbit

Question 18

Julian Calendar

Answer 18

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

Question 19

Latitude

Answer 19

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

Question 20

Local siderial time

Answer 20

the hour angle of ♈︎ for the observer

LST=HA♈︎

Question 21

Longitude

Answer 21

All meridians of longitude curves are great circles (NOT PARALLEL)
Arbitrary zero is the Greenwich Meridian
longitude=(LMST-UT)x15°

Question 22

Maximum elongation

Answer 22

an inferior planet has maximum elongation when its geocentric radius vector is tangential to its orbit.

a superior planet has maximum elongation of 180°

Question 23

Mean solar time

Answer 23

watches and clocks read mean solar time

(discrepancies due to Earth’s elliptical orbit and the ecliptic is not parallel to the celestial equator)

Question 24

Mean sun

Answer 24

moves along celestial equator at same uniform average rate as the sun moves along the ecliptic.

M and S together at the equinoxes

Question 25

Mean time

Answer 25

concept created using mean sun to have a uniform measure of time throughout the year

Question 26

Nautical mile

Answer 26

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)

Question 27

North Celestial pole

Answer 27

The north pole of the Earth projected onto the celestial equator.

Question 28

Obliquity of the ecliptic

Answer 28

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

Question 29

Observer's horizon

Answer 29

the projection of the observer's terrestrial horizon. the curve where the plane perpendicular to the line to the zenith

Question 30

Observer's meridian

Answer 30

the great circle arc that passes through the NCP and the observer's zenith

Question 31

Opposition

Answer 31

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

Question 32

Phase angle

Answer 32

Φ defined at the planet, by the Sun-Earth-Planet system

Question 33

Quadrature

Answer 33

sets the value of planetary elongation to be 90 degrees

Question 34

Retrograde Motion

Answer 34

At opposition, the planet appears to be moving in the direction opposite to its actual orbital motion when viewed from the Earth

Question 35

Rise

Answer 35

non-circumpolar stars rise when it crosses from below the observer's horizon into the visible sky

Question 36

Set

Answer 36

non-circumpolar stars set when it crosses below the observer's horizon

Question 37

Siderial month

Answer 37

the moon completes its orbit around the Earth in 27.32 days relative to the distant stars

Question 38

Siderial year

Answer 38

the time it takes Earth to complete one full orbit around the sun when viewed from distant stars

Question 39

Spherical Sine formula

Answer 39

SinA/sina=SinB/sinb=SinC/sinc

Question 40

Small circles

Answer 40

circles which do not have the same radius as the sphere

Question 41

Solar time

Answer 41

The non-uniform time recorded by the sundial

Question 42

Solstice

Answer 42

The points where the sun has maximum/minimum declination (21st June and 21st December)

Question 43

Superior planet

Answer 43

planets whose orbit lies outside the Earth's orbit

Question 44

Synodic period

Answer 44

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.

Question 45

Topocentric

Answer 45

observer at the centre of the universe

Question 46

Transit

Answer 46

when the star crosses the observer's meridian

Question 47

Tropical year

Answer 47

the period from 1 vernal equinox to the next, comprising 365.2422 mean solar days

Question 48

Tropics

Answer 48

the lines of latitude +/-ε define the tropics of cancer and capricorn

Question 49

Zenith

Answer 49

the point directly over-head of the observer.

Question 50

Zenith distance

Answer 50

the distance to the star as measured from the zenith down towards the horizon Zenith distance=90 degrees - a

Question 51

Zodiac

Answer 51

13 constellations make up the zodiac sun appears to move steadily eastwards around the ecliptic, passing through each of the constellations.