## Info

The conversion of azimuth and altitude to hour angle and declination. Figure 8.9. The conversion of azimuth and altitude to hour angle and declination. The required celestial sphere is shown in figure 8.9 where X is the body's position. In spherical triangle PZX, we see that we require to find arc PX and angle ZPX. We calculate PX first of all, using the cosine formula because we know two sides PZ, ZX and the included angle PZX. cos PX cos PZ cos ZX + sin PZ sin ZX cos PZX sin 8 sin...

## Ser

And, by rounding off the figures, the equation expressing the limiting magnitude of the telescope may be written as where D is expressed in mm. Thus, in practice, the limiting magnitude for a 500 mm telescope is likely to be about 14-5. Half a magnitude has been 'lost' due to the telescope's imperfect transmission. Equation (17.14) is again not a hard and fast law, however, as each telescope must be treated individually and also the limiting magnitude will depend on the observer to some extent....

## Millimetre astronomy

At high sites where the amount of precipitable water vapour is particularly low, several atmospheric windows open in the sub-mm range of the electromagnetic spectrum. Radiations from space at these wavelengths are related to the spectral emissions of molecules within interstellar dust clouds (see section 15.7.4). For this spectral domain, it is more usual to express the radiation in terms of frequency rather than wavelength. The frequency range for this newly open window is from about 25 to...

## Position on the Earths surface

To illustrate these concepts we consider the Earth. A point on the surface of the Earth is defined by two coordinates, longitude and latitude, based on the equator and a particular meridian (half of a great circle) passing through the North and South Poles and Greenwich, England. The equator is the great circle whose poles are the North and South Poles. The longitude, X, of the point is measured east or west along the equator. Its value is the angular distance between the meridian passing...

## Latitude And Zenit Distance Of A Star

8.14 The ecliptic system of coordinates This system is specially convenient in studying the movements of the planets and in describing the Solar System. The two quantities specifying the position of an object on the celestial sphere in this system are ecliptic longitude and ecliptic latitude. In figure 8.19 a great circle arc through the pole of the ecliptic K and the celestial object X meets the ecliptic in the point D. Then the ecliptic longitude, X, is the angle between T and D, measured...

## Instantaneous phenomena

During the day a variety of phenomena may be seen. In a particular direction lies the Sun, so bright it is impossible (and dangerous) to look directly at it. In general, the sky background is blue. The Moon may also be visible, having a distinct shape though certainly not circular. If the Sun has just set or if dawn is not far away, there is sufficient daylight to see clearly. We call this condition twilight. On the horizon opposite to the twilight glow, a dark purple band is sometimes seen....

## Detectors for optical telescopes

We have seen earlier that the Earth's atmosphere has a window which allows transmission of electromagnetic radiation with a range of frequencies, the centre of the band being close to the peak sensitivity of the eye. The eye is not sensitive to all of the frequencies which arrive at the bottom of the atmosphere in this band. Energy in the form of near ultraviolet and infrared radiation is arriving from space and penetrates down to ground-level but is not sensed by the eye. It can, however, be...

## The reduction of positional observations I

In general, astronomical observations of an object's position undergo a process of reduction. This procedure removes known instrumental errors and other systematic effects in order to provide data about the celestial body that is as objective as possible. Such reduced observations, independent of the observer's position, are then suitable for catalogue purposes or for comparison so that changes in the body's position with time can be derived. The raw observations may be the altitude (or zenith...

## Problems Chapter

Note Take the length of the sidereal year to be 365-25 days. Assume all orbits are circular and coplanar, unless otherwise stated. 1. A planet's elongation is measured as 125 . Is it an inferior or superior planet 2. The sidereal period of Mercury is 88 days. What is its synodic period 3. What is the maximum possible elongation of Venus, given that its distance from the Sun is 0-723 AU 4. The synodic period of Jupiter is 398-9 days. What is its sidereal period 5. The heliocentric distance of...