Celestial Sphere

Demonstrate an understanding of the terms 'right ascension' and 'declination'

If you have studied the Earth section you will know the Sun appears to cross the celestial equator along the ecliptic at the vernal equinox. Typically this happens around March 21st. It is called the First Point of Aries but actually takes place in Pisces (!). It changes every year because of the slight wobble in Earth's axis.

All celestial bodies rise in the east and set in the west. When a body such as the Sun or a star reaches its highest point in the sky it is called its culmination. This takes place due south and so we can say it passes, or transits the meridian twice a day. Once we can see it above the horizon (upper transit), the other time we can not as it is below the horizon (lower transit).

The problem with this is that a stars' position changes all the time and differs from location to location. For this reason we use Right Ascension and Declination.

Right Ascension is similar to longitude. On a map of the world we can say that Bristol lies at 51 degrees north (latitude) of the Equator and 2° west (longitude).

Understanding the latitude is easy-it is the number of degrees from the equator from +90° at the North Pole to 0° at the equator and -90° at the South Pole.

Understanding the longitude is more difficult. Our 2° west lies from the Greenwich meridian which dissects London. This is an artificial line created by international agreement.

Right Ascension is the equivalent of longitude in space. We need a marker that everyone on Earth can use.

So if a star lies on a line between where the Sun crosses the Celestial Equator in spring and north (or south) then it will be said to lie at 0° RA (Right Ascension). If the star was on a meridian on the opposite side of the celestial sphere then it may be located at 180° RA.

To find 0° RA is not easy as the most obvious and prominent stars nearby will not be visible throughout the year for everyone on Earth, and sadly for us, the sky does not come with a built-in protractor or compass.

RA sweeps eastwards so anything east of 0° RA is nearer 0. Westwards is nearer 359°. It is measured in hours, minutes (') and seconds ("). 15° is equal to 1 hour of RA.

Declination is a lot easier to understand. It means the latitude of a star in the sky measured from the Celestial Equator which is an extension of our own Equator. If the star is given a declination of +45° then you would see it from the North Pole at 45° in the sky.

The Sun, Moon and planets do not have fixed RA or Dec as they are too close to Earth compared to the stars.

If you are in London (51° latitude) and the star you want to find is declination 90° then it would appear at 51° in the sky. You could see any star with a declination of between 90 and 39° north (90-51).

Most stars have their right ascension and declination published. These are usually abbreviated to RA and DEC. Star maps and planispheres feature gridlines for the observer to follow. In a star map you would see a star listed in this format: Arcturus: RA: 14 15 40 DEC: +19 10 57.

Altitude and Azimuth

You may hear these terms. From any location, we can measure how high in the sky an object is (altitude) in degrees from 0 (horizon) to 90 (zenith). We can also measure its direction from north (0 to 359°). The problem with this is that a star's position changes all the time and from location to location. For this reason we use Right Ascension and Declination.

 

Drag & Drop
Questions

Explain the terms Right Ascension and Declination.

Summary

Right Ascension

How far an object is along the celestial equator from the first point of Aries.

Declination

How far north and south an object is from the Celestial Equator

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