A Telescope is an instrument used for viewing objects at very large distances. Celestial Telescopes are used to look at objects in the sky such as the moon and other celestial objects. Terrestrial Telescopes are used view objects on earth such as mountains, ships and buildings located faraway.

Based on their construction, Telescopes can be divided into two categories, viz. reflecting telescopes and refracting telescopes. The Refracting telescope uses lenses while the reflecting telescope uses mirrors. Refracting telescopes are generally for making viewing objects on earth while reflecting telescopes are used for astronomical observations.

Principle of Operation:
A Telescope is constructed using an objective that gathers light from a faraway source and an eye-piece that forms an image that the eye can see. The objective is a lens in case of a refracting telescope or a mirror in case of a reflecting telescope.

The objective gathers the light from the object to be viewed and focuses it on a point. The second lens the eye-piece collects the light from the focal point of the objective and enlarges it. The magnifying power and the resolution of the telescope are dependent on the choice of the lenses

Famous Telescopes:
Telescopes come in a wide range of sizes and shapes. Small ones can be bought from the toy store for a few dollars while large telescopes weigh many tons and cost millions of dollars. Large telescopes such as the Hubble space telescope.

Besides optical telescopes, most modern astronomical observatories have telescopes which collect all types of electromagnetic radiation such as radio waves, ultraviolet light and create images. The Hubble telescope has an aperture size of 2.4 metres. Other notable telescopes include the 5 meter Hale telescope at the Palomar Observatory.

Important terms related to the telescope:
Resolving power of a telescope: The resolving power of a telescope is the ability of the telescope to differentiate between two different points of the image. Telescopes with higher resolving power produce higher quality images.

Magnifying power of the telescope:
This is the ability of the telescope to magnify the image. The magnifying power is the ratio of the focal lengths of the objective lens and the eye-piece. Therefore by varying the eye-piece lens any magnification could be achieved. However, magnification alone does not produce good quality images. Excessive magnification can result in blurred images.

Aperture: An aperture is a small opening that allows the entry of light. In telescopes, the term “aperture” generally refers to the size of the objective lens which collects the light from the distant object.

Tips for Buying a telescope:
Telescopes are popular amongst amateurs interested in astronomy. A good refracting telescope is ideal for making for looking at the sky. While choosing a good telescope, it is important that you look at the size of the aperture or the objective lens along with the magnification. Many people believe that magnification is all that is necessary to obtain a good image. It is also necessary for a telescope to have a good resolution to deliver.

Besides looking at the optics, it is also necessary to look at the mountings of the telescope along with the portability if you plan to make observations from different places. Read More

Deep in the Jordanian desert, surrounded by towering mountain ranges, lie the ruins of an ancient city that fascinates archaeologists and tourists alike as much with its unique appearance and construction as with its beauty and grandeur. The city is Petra, ancient capital of the Nabateans – a Semitic people. Petra is famous for its buildings – temples, monasteries, churches, amphitheatres that are carved into solid rock.

As remarkable as the ability of the Nabateans to construct buildings out of rock was their creation of a complex system of hydraulic engineering. Located in the middle of a desert and surrounded by mountains, the Nabateans were conscious of the importance and value of water. A complicated system of dams, cisterns, conduits stands testament to the ingenuity of these ancient people. With this clever innovation, water from the flash floods which would ravage the place could be stored for use throughout the year. Water in the desert, being a precious commodity was also traded, bringing in revenue. Thus, Petra was able to transform itself into an artificial oasis in the middle of the desert.

Tradition holds that the earliest known inhabitants of Petra were the Edomites. The Nabateans, scholars hypothesize, invaded the region, displacing the Edomites. Establishing themselves in the region, they managed to hold on despite political turmoil and conflict in their neighborhood. They seem to have been skilful in the art of trade and commerce, taking advantage of the strategic location on ancient commercial routes. For, soon Petra became an important trading post in the fabled spice route. The architecture of the monuments testifies to an influence of many lands. A confluence of native building styles with Greco-Roman architecture produced a grand and imposing style of construction.

The name ‘Petra’ is derived from the Latin word petra which means stone. This was how the city was known to the outside world. It is still unclear what the inhabitants called their city. Scholars believe that the mention of a city called Rekem in the Dead Sea Scrolls refers to Petra.

In 64-63 BC, the Roman General Pompey conquered the Nabateans and Petra came under Roman rule. The Romans, however, allowed the Nabateans to be independent in exchange for tribute. This status continued until 106 AD when the Roman emperor Trajan annexed the region into the Roman Empire. The Romans called the region Arabiae Petrae and retained Petra as its capital. The decline of the city seems to have started a century later. The exact cause for the collapse of Petra is unknown. It is generally believed that a combination of factors such as the development of rival trade centers in the region and external invasions may have been responsible for the decline of this wondrous city. The city all but disappeared from the glare of the outside world until 1812, when the Swiss explorer Johann Ludwig Burckhardt discovered it.

Some of the earliest religious buildings are temples built in honor of local gods and goddesses and deified kings. Christianity reached Petra in the 4th century AD. A number of churches have been discovered. However, it was supplanted by Islam, which reached the region in the 7th century AD.

Efforts are on by the Jordanian Government and UNESCO to preserve this historic treasure for future generations. Petra is currently mentioned on the World Heritage List. Many restoration missions and photogrammatic surveys have been conducted at this site. A comprehensive plan is being designed to protect this site and to develop a sustainable economic model for tourism with the involvement of the local population.

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