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Astronomy 1 > Positional Astronomy > Flashcards

Positional Astronomy Flashcards

1
Q

Altitude

A

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

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2
Q

Azimuth

A

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

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3
Q

Celestial equator

A

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

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4
Q

Circumpolar

A

Stars that remain above the horizon at all times

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5
Q

Conjuction

A

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

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6
Q

spherical cosine formula

A

cosa=cosbcosc+sinbsinccosA

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7
Q

Declination

A

The equivalent of latitude on the celestial sphere

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8
Q

Direct motion

A

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.

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9
Q

Diurnal motion

A

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

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10
Q

Ecliptic

A

apparent path taken by the sun

titled at ε with respect to celestial equator

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11
Q

Elongation

A

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

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12
Q

Equation of time

A

EqT=RAMS-RAS

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

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13
Q

Equinox

A

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

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14
Q

Geocentric

A

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

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15
Q

Great Circles

A

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

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16
Q

Hour Angle

A

equivalent to longitude on the celestial sphere

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17
Q

Inferior planet

A

planets whose orbits lie inside the Earth’s orbit

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18
Q

Julian Calendar

A

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

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19
Q

Latitude

A

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

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20
Q

Local siderial time

A

the hour angle of ♈︎ for the observer

LST=HA♈︎

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21
Q

Longitude

A

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

22
Q

Maximum elongation

A

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

a superior planet has maximum elongation of 180°

23
Q

Mean solar time

A

watches and clocks read mean solar time

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

24
Q

Mean sun

A

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

M and S together at the equinoxes

25
Q

Mean time

A

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

26
Q

Nautical mile

A

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)

27
Q

North Celestial pole

A

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

28
Q

Obliquity of the ecliptic

A

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

29
Q

Observer’s horizon

A

the projection of the observer’s terrestrial horizon.

the curve where the plane perpendicular to the line to the zenith

30
Q

Observer’s meridian

A

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

31
Q

Opposition

A

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

32
Q

Phase angle

A

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

33
Q

Quadrature

A

sets the value of planetary elongation to be 90 degrees

34
Q

Retrograde Motion

A

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

35
Q

Rise

A

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

36
Q

Set

A

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

37
Q

Siderial month

A

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

38
Q

Siderial year

A

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

39
Q

Spherical Sine formula

A

SinA/sina=SinB/sinb=SinC/sinc

40
Q

Small circles

A

circles which do not have the same radius as the sphere

41
Q

Solar time

A

The non-uniform time recorded by the sundial

42
Q

Solstice

A

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

43
Q

Superior planet

A

planets whose orbit lies outside the Earth’s orbit

44
Q

Synodic period

A

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.

45
Q

Topocentric

A

observer at the centre of the universe

46
Q

Transit

A

when the star crosses the observer’s meridian

47
Q

Tropical year

A

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

48
Q

Tropics

A

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

49
Q

Zenith

A

the point directly over-head of the observer.

50
Q

Zenith distance

A

the distance to the star as measured from the zenith down towards the horizon

Zenith distance=90 degrees - a

51
Q

Zodiac

A

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

Astronomy 1 (6 decks)

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