Astronomy

Altitude

Angle of an object above observer’s horizon

Autumnal equinox

Point where Sun crosses the equator going South

Azimuth

Angle of an object measured from North around the horizon

Celestial sphere

Imaginary sphere carrying stars that spins around the observer

Declination

Angle of an object measured from the celestial equator

Ecliptic

Great circle path on celestial sphere followed by the Sun

Equator

Great circle perpendicular to the North-South polar axis

Great circle

Largest possible circle that can be drawn on a sphere

Meridian

Imaginary line in the sky over an observer running North-South

North Celestial Pole

Point in North where celestial sphere’s spin axis meets the sphere

Opposition

When planets are on opposite side of the sky to the Sun

Right Ascension

Angle of an object measured around the celestial equator

Sidereal day

Time taken for Earth to rotate once relative to the “fixed” stars

Sidereal time

Time scale locked to the fixed stars

Solar day

Time taken for Earth to rotate once relative to the mean Sun

South Celestial Pole

Point in South where celestial sphere’s spin axis meets the sphere

Summer solstice

Most northerly point of Sun’s path on the celestial sphere

Synodic day

Alternative form of “solar day”, which generalises to any planet

Winter solstice

Most southerly point of Sun’s path on the celestial sphere

Vernal equinox

Point where Sun crosses the equator going North

Zenith

Point directly overhead an observer

What are the horizon coordinates?

• Altitude (measured in degrees from the horizon)

• Azimuth (angle measured from north through east)

What line do objects cross at their highest point in the sky?

The meridian

What is the formula for maximum altitude/meridian transit?

Altitude = 90 − |ϕ−δ|

(phi is latitude, delta is declination)

What is the relation between the length of an object ℓ at distance d which subtends an angle α? Use small angle approximation

ℓ ≈ αd

If distance d is expressed in parsecs and the parallax angle p in arcseconds, what is their relationship?

d = 1/p

What is the flux of an object, and what are it’s units?

Flux is the apparent brightness of an object as seen from a specific viewpoint (like Earth). It’s units are Wm^-2.

What is the absolute magnitude of an object?

It’s (apparent) magnitude if it were observed from a distance of 10pc.

What does a reflecting telescope use to focus light?

A mirror

What does a refracting telescope use to focus light?

A lens

What is the surface brightness of an object?

The flux at Earth from the object per unit solid angle (steradian) of sky.

How many steradians cover the entire sky?

4π

What is the focal ratio/f-ratio and what does it determine?

f/D (focal length over the diameter of the lens)

Determines the length of exposure needed when taking images.

e.g. “f/8” for f/D = 8

What is angular resolution?

The smallest angle between two stars where one can still distinguish them as two rather than a merged single star.

What is the spectrum of an object?

The distribution of it’s flux with wavelength