Watching the summer sky(II)

following the charts

Charts of the night sky is a must for the naked eyed stargazers. While using the astronomical charts you may come across certain terminologies. And, the may not sound familiar to you.

The following glossary might be helpful in following those charts or guide map brought out by your local skywatchers' associations.

Constellation: Constellation is the group of some bright stars. In the early days the constellations were believed to have influence on daily life. Thus most of them represent mythological humans or animals. This is formed by joining an imaginary line following a connect-the-closest-visible-dot order. Though it often takes a lot of imagination to see the supposed picture. The constellations appear different from different latitudes. Also their appearance vary from time to time due to earth's annual rotation. So the sky overviewing one constellation at summer nights will shift away during the winter nights. And this apparent turning of the heavens is around a point midway up the northern sky, marked by the Polaris. Thus constellations near the Polaris are visible all through the year to people in the Northern hemisphere. Constellations' visible stars are numbered in order of brightness with letters of the Greek alphabet: alpha, beta, gamma, delta, epsilon, zeta, eta, theta, iota, kappa, lambda, mu, nu, omicron, pi, rho, sigma, tau, upsilon, phi, chi, psi, omega. In most constellations the brightest star is called Alpha, the second Beta, and so on.

Parallax: The distances of the nearest stars are measured by means of their parallaxes. It means the change in the direction of an object as the observer moves around. If you hold up your finger and look at it while moving your head back and forth. Similarly, as the Earth goes around the Sun every year the nearby stars seem to move back and forth against the background of more distant stars. The amount by which a star appears to move back and forth every year is called parallax. Thus, the closer a star, the larger is the parallax. Accordingly, the largest parallax belongs to the nearest star Proxima Centauri. The parallax of it is almost like a dot.

Parsec: Astronomers use a unit of length, called parsec to measure the distance of stars. One unit is about 200,000 times the distance from the Earth to the Sun, or, just under 20 trillion miles. It is chosen because it is the distance at which the parallax of a star is exactly one second of arc, or 1/36000 of a degree. The nearest other star to the Sun is just over a parsec away. While the neighboring ones are a parsec or two apart.

Magnitude: The comparative brightness of stars, as we see them, is called magnitude. The Greeks referred to the brightest of the stars as the first magnitude and so on. Modern astronomers use the same system with a different name and aim to achieve better precision. According to this scale, as we move up from negative to positive the brightness of the stars gets reduced.

Sirius: The brightest star with a magnitude of -1.4 in the constellation Canis Major. Visible above 50° north latitude. Usually most prominent during summer's end dawn.

Polaris (Polestar): The brightest star in the constellation of  Ursa Minor. Polaris  in Latin is called, Lakota Wichapi Owangila, meaning the star that stands in one place. This attribute has made it  a useful identifier in the night sky to people in the North.

Pollux: The brightest star of magnitude -1.1 in the constellation Gemini. Castor-Pollux "Twins" of Gemini seem to indicate an actual change in brightness. Pollux is now Gemini's only first magnitude star. While Castor was equally bright up to about 400 years ago. That's why this constellation was called "Twins". Now the twins look somewhat different.

Milky Way: Most of the stars we see in the sky at night are within 100 or, 200 parsecs of us. But this is just a tiny part of the whole stellar system that we live in. The remaining portion shows up only as a faint irregular white band around the sky. It's visible only when the sky is clear without any moonlight. This system is called the Milky Way. Our Milky Way is just one among many. This is in the shape of a great disc. The disc is 30,000 parsecs in diameter and from 500 to 2,000 parsecs in thickness. Our solar system is located nearer to the edge of the disc.

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