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2. The Celestial Sphere Goals : 1. Gain familiarity with the basic co-ordinate systems used in astronomy. 2. Tackle simple problems in practical astronomy involving timekeeping and star positions. 3. Examine the timekeeping systems in current use. Astronomical Co-ordinate Systems:

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2. The Celestial Sphere


1. Gain familiarity with the basic co-ordinate systems used in astronomy.

2. Tackle simple problems in practical astronomy involving timekeeping and star positions.

3. Examine the timekeeping systems in current use.


Astronomical Co-ordinate Systems:

All co-ordinate systems constructed on spheres are defined by a fundamental great circle (FGC) and a reference point (RP) on the FGC.

All co-ordinates are angles measured:

(i) between great circles perpendicular to the FGC, or

(ii) between small circles parallel to the FGC.

The FGC has two poles, and the RP is defined in a variety of ways, which accounts for slight differences from one system to another.


Terrestrial Co-ordinates.

FGC = Earth’s equator: poles

the North Pole and South Pole.

RP = crossing point of equator

by the Greenwich meridian.


Longitude = angle measured east and west from the Greenwich meridian. Longitude meridians are great circles.

Latitude = measured north and south (not plus or minus) from the Equator. Latitude parallels are small circles.

Examples: Halifax. 63º 36'.0 W, 44º 36'.0 N

Vancouver. 123º 04'.2 W, 49º 09'.0 N


Horizon System.

FGC = horizon, with poles the zenith and nadir.

RP = north point.


azimuth = angle measured through east from 0º to 360. Azimuth circles are great


altitude = measured from horizon towards zenith (positive) or nadir (negative) from +90º to –90º. Alternate: zenith distance,

z = 90º – altitude.


Meridian = NS line running through zenith.

Prime vertical = EW line running through zenith.

Use. Airport runways are designated by azimuth  10°, i.e. runway 32 aligns along azimuth 320° magnetic. Air mass, for correction of photometry, is calculated from zenith distance z.

Example Question. In what directions does runway 05/23 run?

Solution: Given the name of the runway, 05/23, it must run along azimuth directions 50° (northeast) and 230° (southwest), which are 180° apart.


Equatorial System.

FGC = celestial equator (CE, projection on the sky of Earth’s equator), with poles the north and south celestial poles, NCP and SCP.

RP = intersection point of meridian with CE (observer-oriented), or vernal equinox γ (sky-oriented).


declination = angle measured

north or south of CE from 0º

to +90 and 90° (δ).

hour angle = angle measured

west of meridian (HA), or

right ascension = angle measured

eastward from vernal equinox (RA).


From relations for angles associated with parallel lines, 90° – θ = 90° – , i.e. θ = .




1h exposure


The orbit of Earth about the Sun and the 23½° obliquity of the ecliptic (its angle relative to the normal to Earth’s orbital plane) give rise to Earth’s seasons.


RA() and δ() during the year are defined by the apparent motion of the Sun in the sky along the ecliptic = Sun’s apparent path, and can be calculated directly or from tables.


Solar day (24h) is rotation of Earth relative to Sun, sidereal day (23h 56m) is rotation of Earth relative to stars.


HA and RA are measured in temporal units and are equivalent to angles. On the celestial equator: 1h = 15°, 1m = 15', and 1s = 15″, with the equalities changing by cos δ with increasing declination.

Because of their link to timekeeping, HA and RA are tied directly to sidereal (star) time and apparent solar time.

Sidereal time (SidT)= HA(γ)

Apparent solar time = HA() + 12h

Now, HA(γ) = HA(*) + RA(*) = HA() + RA()

Thus, SidT = HA() + RA()

= Apparent solar time  12h + RA()


By geometry and algebra, Sidereal Time = HA(γ)

But HA(γ) = HA(*) + RA(*)

So Sidereal Time = HA(*) + RA(*)


Useful values:

Vernal Equinox, March 20:

RA() = 0h, δ() = 0°

Summer Solstice, June 21:

RA() = 6h, δ() = +23½°

Autumnal Equinox, September 23:

RA() = 12h, δ() = 0°

Winter Solstice, December 22:

RA() = 18h, δ() = 23½°

annual insolation

The actual dates of the equinoxes and solstices slowly change with time. They were March 25, June 25, September 25, and December 25 when Julius Caesar modified the original Roman calendar system (Julian Calendar) in 46 BC.


An example.

1. Towards what directions on the co-ordinate axes must an equatorial telescope be set in order to point it towards Betelgeuse, RA = 5h 56m, δ = +7° 25' at 5h sidereal time?



By definition, SidT = HA(γ) = HA(*) + RA(*)

 5h = HA(*) + 5h 56m

And so HA(*) = 5h – 5h 56m = –0h 56m

The hour angle setting of the telescope should be set to –0h 56m, i.e. 56m east of the meridian.

And the declination setting of the telescope should be set to +7° 25'.


Another example.

2. Show that apparent solar time (AST) and sidereal time (SidT) are identical on the date of the autumnal equinox.



By definition, SidT = HA(γ) = HA(*) + RA(*)

The Sun is also a star, so SidT = HA() + RA()

But AST = HA() + 12h

And RA() = 12h at the Autumnal Equinox.

So at the Autumnal Equinox,

SidT = HA() + RA() = HA() + 12h = AST

i.e. Sidereal Time and Apparent Solar Time are identical on the date of the Autumnal Equinox.


3. When is the best time of year to observe the stars of Orion, RA = 5½h?


The optimum time for observing any object is when it lies on the observer’s meridian at local midnight, which corresponds to 0h local apparent solar time (LAST).

i.e. LAST = HA() + 12h = 0h (midnight)

So HA() = 0h  12h = 24h  12h = 12h

Orion is then on the meridian, so local sidereal time = RA(*) = 5½h =HA() + RA()

 RA() = 5½hHA() = 5½h12h = 29½h12h = 17½h

The Sun is at RA = 17½h approximately one week prior to the winter solstice, i.e. around Dec. 15.


Ordering the Planets Outwards

Object Motion relative to the Stars

Stars infinite

Saturn 29.30 years

Jupiter 11.86 years

Mars 1.88 years

Sun 365¼ days

Venus 225 days

Mercury 88 days

Moon 27½ days


Saturn governs the 1st hour of the 1st day, Jupiter the 2nd hour, Mars the 3rd hour, etc., and Mars the 24th hour. The Sun then governs the 1st hour of the 2nd day, the Moon the 1st hour of the 3rd day, Mars the 1st hour of the 4th day, Mercury the 1st hour of the 5th day, Jupiter the 1st hour of the 6th day, and Venus the 1st hour of the 7th day. The days of the week are therefore:










Apparent solar time is defined by the passage of the Sun across the sky, but civil time is more closely related to the motion of the mean Sun, a fictitious object, across the sky.

Mean solar time = HA(mean Sun) + 12h

The mean Sun differs from the true Sun in the following way. The true Sun travels along the ecliptic at a rate that varies according to the distance of Earth from the Sun. The mean Sun travels along the celestial equator at a uniform rate.

Additional complications arise from the use of time zones and daylight saving time.


Length (days)

Julian Gregorian

Month Calendar Calendar

January 31 31

February 29 28

March 31 31

April 30 30

May 31 31

June 30 30

Quintilus (July) 31 31

Sextilus (August) 30 31

September 31 30

October 30 31

November 31 30

December 30 31

important calendar dates
Important Calendar Dates

Calendar Event Julian Modern Vernal Equinox March 25 March 20 Summer Solstice June 25 June 21 Autumnal Equinox Sept. 25 Sept. 23 Winter Solstice Dec. 25 Dec. 22


The year length varies according to the calendar system, which has changed from lunar calendars, through luni-solar calendars, to solar calendars, such as the Julian Calendar, Gregorian Calendar, and current modified Gregorian Calendar.

Variable star studies normally cite observations according to the Julian Date, JD, measured as the number of sequential days from noon, UT, on January 1, 4713 BC (named by Joseph Scaliger after his father Julius Scaliger), or, better yet, HJD = Heliocentric Julian Date (corrected to the barycentre of the solar system). Another term, modified Julian Date, MJD = JD  2400000.5, is occasionally used.


Ecliptic System.

FGC = ecliptic, with poles the north and south ecliptic poles, NEP and SEP.

RP = vernal equinox γ.


celestial (or ecliptic)

longitude, λ= angle

measured eastward

from γ from 0º to 360.

celestial (or ecliptic)

latitude, β = angle

measured from ecliptic.

The system is useful for studies of solar system objects.


Galactic System.

FGC = Galactic equator (GE), defined by the Milky Way, with poles the north and south Galactic poles, NGP and SGP.

RP = direction to the Galactic centre (GC), defined by Sgr A*.


Galactic longitude, l =

angle measured

Eastward from GC

from 0º to 360.

Galactic latitude, b =

angle measured north or

south of GE from 0º to

+90 and 90°.


Precession of the Equinoxes.

Earth’s axis of rotation precesses relative to the perpendicular to its orbit because of gravitational influences by the Sun and Moon, but not in the fashion implied by the Wikipedia figure below. The sense of precession is

actually opposite the sense

of Earth’s rotation. The

period is ~25,725 years.

A top’s



Precession and

the location of

the NCP. Note

that the NCP

was near the

bright star

Thuban near

2700 BC, when

the pyramids

were built, and

was once near

Vega, a name

that means



Astronomical Terminology

Zenith. The point in the sky directly overhead.

Nadir. The point directly beneath one’s feet.

Azimuth. A measurement of angle increasing from north through east.

Altitude (astronomical). A measurement of angular distance from the true horizon upwards.

Ecliptic. The great circle in the sky along which the Sun appears to move because of Earth’s orbit about it.

Right Ascension. A celestial co-ordinate like longitude on Earth, increasing eastwards.

Declination. A celestial co-ordinate like latitude on Earth, measured from the celestial equator.

Celestial Equator. The projection on the celestial sphere of Earth’s equator.

Celestial Sphere. The imaginary sphere centred on the observer upon which the stars appear to be projected.


Astronomical Terminology (continued)

Diurnal. = daily (once a day).

Insolation. The amount of sunlight falling on Earth’s surface.

Constellation. A group of conspicuous stars designated by ancient star gazers.

Zodiacal Constellation. A constellation lying in the band of sky around the ecliptic, where the Moon and planets are always found.

Solstice. Time of greatest or smallest declination for the Sun.

Equinox. Time when the Sun crosses the celestial equator. (Vernal = spring)

Stellar Aberration. The apparent displacement in a star’s location in the sky of at most 20½ seconds of arc resulting from Earth’s orbital motion about the Sun at a speed of 30 km/s.