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Our view of the sky

Local reference lines

You can describe the position of objects in the sky with respect to your particular location. The point directly above you is the zenith and the celestial horizon is the circle 90° away from the zenith.

The four compass points and zenith
The four compass points and zenith
Altitude and azimuth
Altitude and azimuth
Altitude and azimuth coordinates describe the position of a star or planet at a given time and place. Click on the images to enlarge

The celestial meridian is the circle running through the zenith and the due north and due south points on the horizon. If you look towards the south in the northern hemisphere, the rotation of the Earth makes the stars appear to move from east to west. The stars culminate when they reach their highest point as they cross or transit the meridian.

To describe the location of a star or planet with respect to an observer, we can use altitude and azimuth. A star's altitude changes from 0° at the horizon to 90° at the zenith. The azimuth of a star is measured as a direction around the horizon, where 0° is due north, 90° is due east, 180° is due south and 270° is due west. Therefore if a star is halfway between the horizon and the zenith and is in the southeast, it would have an altitude of 45° and an azimuth of 135°.

The big problem with local coordinates is that they are only correct for a particular time and place. Someone to the east, west, north or south will see a different sky and the position of the object will be different. Even staying in the same place, the sky will change as the Earth rotates. To overcome this difficulty we need to introduce a system of coordinates which are independent of time and place - celestial coordinates.


Celestial coordinates

From Earth, the eye perceives all stars and planets to be equally remote. They can be depicted as being on a celestial sphere that spins westward due to the rotation of the Earth.
The celestial sphere
From Earth all stars appear to be equally distant, irrespective of how far away they really are
The spinning celestial sphere
As the Earth rotates from west to east, the celestial sphere appears to rotate from east to west

Click on the images to enlarge


The north and south celestial poles (NCP and SCP) lie above the north and south poles of the Earth, whilst the celestial equator is above the equator of the Earth. The position of a star on this sphere is then measured using a system similar to latitude and longitude on the surface of the Earth.

Declination (Dec.) is analogous to latitude. It measures a star's position north or south of the celestial equator. Declination runs from +90° at the NCP through 0° at the celestial equator and down to -90° at the SCP.

Right ascension (RA) is similar to longitude. It is measured eastwards from the north-south line containing the first point of Aries - the position of the Sun at the vernal equinox. RA is measured in hours, with 24h making a complete eastwards circle around the sky from 0h at the vernal equinox. At the celestial equator, each hour of RA corresponds to an angle of 15°.

The RA and declination of a star will be the same all over the world and for a long period of time as stars only change their position noticeably over many centuries.


Latitude and stellar positions
The appearance of the sky changes with latitude. The north celestial pole is very close to and so is marked by the pole star, Polaris. This is only visible from the northern hemisphere and its altitude above the horizon is roughly equal to the latitude of the observer.


Circumpolar stars

From the ground, the stars appear to move in daily circles around the celestial poles as the Earth rotates.

How much of each circle is above the horizon depends on both the latitude of the observer and the declination of the star. For example, from 50°N latitude, stars within 50° of the NCP (i.e. with a declination greater than +40°) are circumpolar - they never set.

Circumpolar stars
Stars on the inner circle (including the Plough) are circumpolar - at their lowest point they remain above the northern horizon. Stars near the outer circle are too far from the celestial pole to be circumpolar - they rise and set each day
Similarly, at the same latitude of 50º N, stars which are never less than 90° away from the zenith will never rise. The zenith will be at declination +50° and a star reaches its maximum altitude when it culminates.

In the northern hemisphere, a star reaches its maximum altitude when it culminates or crosses the meridian between the celestial pole and the southern horizon. In the southern hemisphere, a star culminates when it crosses the meridian between the celestial pole and the northern horizon. A star will never rise if it culmination point is on or below the horizon and is 90º from the zenith. For latitude 50º N this would correspond to a declination of -40º.

In general, a star is circumpolar if its distance from the pole is less than or equal to the observer's latitude f. Expressed as a formula, stars of declination d are circumpolar if:

Equation for circumpolar stars (declination is greater or equal to 90-latitude)

where all the angles are measured in degrees.


The Ecliptic

Over one year, the motion of the Earth around the Sun causes the Sun's motion from west to east against the background stars. The line of this path is the ecliptic which is also the name given to the plane of the Earth's orbit around the Sun.

If the ecliptic is plotted on a star chart based on right ascension and declination it appears as a curved line. This is a consequence of the tilt of the Earth's axis with respect to the plane of its orbit around the Sun.

Diagram showing the weaving ecliptic line
Diagram showing the weaving ecliptic line against the right ascension and declination grid of the sky

A - March equinox (First point of Aires)
B - June solstice
C - September equinox (First point of Libra)
D - December solstice


Four points define the seasons and positions of the Sun over the year.

  • Firstly, the Equinox on or around 21 March is when the Sun moves northwards across the celestial equator. At this time the Sun has a declination of 0° and a right ascension of 0h.

  • At the Solstice on or around 21 June the Sun is at its northernmost position in the sky at declination +23.5°, right ascension 6h.

  • 3 months later at the September Equinox the Sun moves southwards across the celestial equator. It has a declination of 0° and a right ascension of 12h.

  • The Sun reaches its southernmost point at the December Solstice when it has a declination of -23.5° and a right ascension of 18h.

Finally the Sun returns to the March Equinox three months later, completing its motion over the year. This motion is a consequence of the orbit of the Earth.

Most of the planets move through a band of sky centred on the ecliptic and known as the zodiac. Thirteen constellations or patterns of stars contain the zodiac: Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Ophiuchus, Sagittarius, Capricornus and Aquarius.


Atmospheric refraction

Earth's atmosphere affects the appearance and apparent position of objects in the night sky. Firstly, the decreasing density of layers of air with altitude causes refraction of the light from objects. This has the effect of making objects appear to be higher in the sky than they really are.

The effect of refraction on the position of the Sun and the times of sunrise and sunset
The effect of refraction on the position of the Sun and the times of sunrise and sunset
The most dramatic example of this effect is at sunset. In the diagram above, if there was no atmosphere then when the Sun is at position A, it would already be below the horizon of the observer.

However, refraction causes the Sun's rays to bend down towards the observer so that it appears to be at point B. This delays sunset by some minutes. Refraction also causes the lower limb of the Sun to be raised more than the upper limb giving it a squashed appearance.

A similar effect occurs with stars and planets.


Scattering of sunlight - why the sky is blue
The scattering of sunlight is responsible for making the sky appear blue in colour. When sunlight is incident on molecules of air, the blue part of the spectrum is sent in a variety of directions and causes the appearance of the sky, whilst the Sun itself appears a yellowish colour as red and yellow light is scattered less.

At sunset this effect is strongest, so the Sun appears red and the sky a deeper blue colour.


Questions to think about

1. Is Mizar (declination +55°) circumpolar from Cairo (31°N)?


2. Will Alpheratz (declination +29°) set if you watch it from Hammerfest (71°N)?


3. Is Acrux (declination -63°) circumpolar from Buenos Aires (latitude 35°S)?

Example
An observer is situated in south London (latitude 51°N). Are the stars a) Capella (declination +46°) and b) Aldebaran (declination +17°) circumpolar?

Using the equation d90–f, a star will be circumpolar if:

a) Capella: is 46° greater than or equal to 90° - 51°?
Answer: 46° is greater than or equal to 39° therefore it is circumpolar from London.

b) Aldebaran: is 17° greater than or equal to 90° - 51°?
Answer: 17° is not greater than or equal to 39° so Aldebaran is not circumpolar from London.


4. What are the zodiac, ecliptic and vernal equinox (on the sky)?


5. A bright planet crossed the sky during 2001. The table below gives the positions it had on particular dates. Plot the positions on a graph at an appropriate scale and estimate the date on which it was closest to the winter solstice.

Date Right Ascension Declination
1 August 16h 58m -26° 52'
8 August 17h 04m -26° 56'
15 August 17h 13m -26° 59'
22 August 17h 24m -27° 1'
29 August 17h 37m -27° 1'
5 September 17h 52m -26° 56'
12 September 18h 8m -26° 47'
19 September 18h 24m -26° 30'
26 September 18h 42m -26° 06'


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