THE THAMES OBSERVATORY.

New Zealand Herald, Volume XXIX, Issue 8986, 17 September 1892, Page 1 (Supplement)

THE THAMES OBSERVATORY.

No. 11.-A DAYLIGHT VISIT.

A beautiful morning, bluo sky, with a few white fleecy clouds. The sun being well up, we wend our way to pay our promised daylight visit to the Thames Observatory. Arriving at Mr. Grigg's warehouse wo notice two persons looking through the window and comparing their watches with a large clock having a long pendulum. Mr. Grigg informs us that this clock is kopt to New Zealand official time, and is placed in. the warehouse instead of the observatory for several reasons, one of which is the convenience of the public, who show their appreciation by the use they make of it. This clock is regulated to gain two or three seconds per week, and it is checked by astronomical observation, it is never allowed to be more than a few seconds fast. A microphone is fitted inside, and is connected by wire with the observatory in the garden, where a telephone receiver reproduces its ticks. Wo pass through, and are again on the concrete floor of the littlo round house. The shutter is removed from the dome, and the telescope turned towards the sun. By certain contrivances the light and heat of that mighty orb are so reduced that we can view it with ease magnified 60 diameters. Its edge appears boiling all round. We ask if they are flames we seo, bub are told no, it is only the effect of the strong air currents which the direct rays of the sun produce. Wo notice several groups of spots on the sun's surface. Those spots have very irregular forms, the central portions being almost black, while the margins are of a much lighter shade. The general appearance of the sun's face is rough, like sandpaper, and near the edge, where there is a spot, are a number of little white patches called faculas. What are these spots? We don't know; probably mighty furnaces or storms in old Sol's atmosphere How big are they? Well, we will try ami > e.isure one of them. So saying the astronomer switches on the telephone, and we hear the ticks of the big clock quite loud. Now count how many ticks while that spot near the middle of the sun crosses the wire in the telescope—one, two, three, four. N'ow count again how long is the sun in crossing it, 136 seconds; then we say, as four is to 130, so is the spot's diameter to the sun's, and the latter being estimated at 865,000 miles, the former will be '25,440. the area of which is, therefore, about SOS millions of square miles. We consider this a large spot, but by no means the largest. There was one visible in February the area of which was calculated as being 2911) million. Do these spots on the sun affect our world ? Yes ; bub at prosent we do not know to what extent. They are often coincident with electrical disturbances here, which cause the magnetic needles to swing out of their usual positions, and not unfrequently hinder the work of telegraphists. There is much interest taken by astronomers in observing and recording these solar disturbances. Bub I see you are examining the telescope, so I must give you the promised description of it. lb is a refractor by Wray, with a 3J-inch object glass, and a number of different eyepieces and various other fittings. The iron Btund which carries ib is securely fixed to the concrete pillar to ensure perfect stability. It revolves on two axes at right angles with each other; they are fitted with graduated circles for indicating the position of the object towards which the tube is turned. One of these axes is exactly parallel with tho axis of the earth, so that the object sighted can bo kept in view by one motion only. Mounted thus, a tele-cope is called an " Equatorial," and when in correct adjustment we can with it find an object otherwise invisible to us, provided we know its exact position at the time. Suppose we look for Sirius. Hereupon the astronomer glance! at a small clock in the observatory, performed some mental calculations, and set the equatorial by its circles. Revolving the dome* that its opening might be opposite the telescope, lie told us bo look through. We saw no thing but sky. He then looked himself, touched a screw, and said, " Look again." We did so, and sure enough, there was the star travelling across quite steadily. This is the brightest of tho stars, and is commonly known as the dogstar. Going through a similar process, and turning the instrument nearer to the sun, ho next showed Venus, which at the pre sent time appears as a crescent, like the moon.

Several stars, whoso names were given, were then shown. To see the stars in bright sunshine was a new experience, and we were surprised to observe the brilliance of their sparkling, which seemed to lose but little through having a bright background instead of a dark one.

Noticing that the clock in the observatory was several hours different from our time, we asked an explanation and were informed that it showed sidereal timo, which corresponds with solar time only once in a year. Sidereal time is commonly used in observatory work and calculations, as being mote convenient than solar time, to which it can readily be converted when necessary. For obtaining the time the sun and stars are observed on the meridian—that is, due north or south. For this purpose a small instrument called a transit is used. We had the pleasure of observing the transit of a star in this instrument. Taking a transit is a very neat observation. The star is seen passing behind several wires, while the seconds shown by the clock are counted and recorded. The star's time being known, the error of the clock is shown at once. The transit necessarily requires to be adjusted with the greatest accuracy, as it is the standard or prime referee of the observatory. Mr. Grigg has set up a little post on a hilltop a mile or two distant, due'north, and has another mark, due south. There is also an instrument called a collimator in the observatory for fixing the north point at night. All the instruments are fitted with " wires" —that iB, spider's webs, selected for their evenness and fineness. The equatorial is fitted with electric lamps for illuminating the wires and circles. The current, which is also available for other purposes, is supplied by a bichromate battery. Mr. Grigg ha 3 made some fair attempts at celestial photography. We saw a series of views of an eclipse of the sun, which took place some time ago, the instantaneous exposures for which were made by actuating an electro magnet. For photographing stars a protracted exposure is necessary, during which time the camera must follow the star's motion. This is accomplished by saddling it to the telescope, and keeping a guide star on the wires of the latter while revolving it.