Astronomy PDF

Constellations Constellations on the Ecliptic. As Earth revolves around the Sun, we sit on “platform Earth” and see the Sun moving around the sky. The circle in the sky that the Sun appears to make around us in the course of a year is called the ecliptic. This circle goes thr...

Constellations Constellations on the Ecliptic. As Earth revolves around the Sun, we sit on “platform Earth” and see the Sun moving around the sky. The circle in the sky that the Sun appears to make around us in the course of a year is called the ecliptic. This circle goes through a set of constellations. The ancients thought these constellations, which the Sun (and the Moon and planets) visited, must be special and incorporated them into their system of astrology. Note that at any given time of the year, some of the constellations crossed by the ecliptic are visible in the night sky; others are in the day sky and are thus hidden by the brilliance of the Sun. Constellations on the Ecliptic The ecliptic does not lie along the celestial equator but is inclined to it at an angle of about 23.5°. In other words, the Sun’s annual path in the sky is not linked with Earth’s equator. This is because our planet’s axis of rotation is tilted by about 23.5° from a vertical line sticking out of the plane of the ecliptic. Being tilted from “straight up” is not at all unusual among celestial bodies; Uranus and Pluto are actually tilted so much that they orbit the Sun “on their side The Celestial Tilt UNDERSTANDING THE STARRY SKY On a clear, dark night the sky looks like a gigantic dome studded with stars. So the ancients believed that the starry sky was a huge sphere turning around Earth. Today we know that stars are remote, and the Earth rotates around its axis (the imaginary line running through its center between the North and South Poles). But the picture of the sky as a huge, hollow globe of stars that turns around Earth is still useful. Astronomers call this fictitious picture of the sky the celestial sphere. “Celestial” comes from the Latin word for heaven. Why do the stars on the celestial sphere appear to move during the night when you observe them from Earth? CONSTELLATIONS Stars appear to belong to groups that form recognizable patterns in the sky. These star patterns are called constellations. There are 88 constellations officially recognized by the International Astronomical Union. Thousands of years ago people named the constellations after animals, such as Leo the Lion or mythological characters, such as Orion the Hunter. More than 2000 years ago the ancient Greeks recognized 48 constellations. 12constellations located around the ecliptic are the constellations of the zodiac whose names are familiar to horoscope readers. List the 12 constellations of the zodiac Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius. Star maps--You can use the star maps outdoors to identify the constellations and stars you see in the night sky and to locate those you want to. observe 40°N latitude CIRCUMPOLAR CONSTELLATIONS In maps we can sees several constellations, near the celestial poles, these are called circumpolar constellations. There are north circumpolar constellations, and south circumpolar constellations. South circumpolar constellations do not rise above the horizon any night of the year. three circumpolar constellations closest to Polaris (the North Star) are Cassiopeia, Cepheus, and Ursa Minor STAR NAMES Long ago, more than 50 of the brightest stars were given proper names in Arabic, Greek, and Latin. The names of bright or famous stars to look for are printed on your star maps with the initial letters capitalized. Today astronomers use alphabets and numerals to identify hundreds of thousands of stars. They refer to each of the brightest stars in a constellation by a Greek letter plus the Latin genitive (possessive) form of the constellation. Usually the brightest star in a constellation is α, the next brightest is β, and so on. Thus, Regulus is called α Leonis, or the brightest star of Leo LOCATION ON EARTH We picture the Earth as a sphere and draw imaginary guidelines on it. All distances and locations are measured from two main reference lines, each marked 0°. One line, the equator, is the great circle halfway between the North and South Poles that divides the globe into halves. The other line, the prime meridian, runs from pole to pole through Greenwich, England. Imaginary lines parallel to the equator are called latitude lines. Those from pole to pole are called longitude lines, or meridians. You can locate any city on Earth if you know its coordinates of latitude and longitude. CELESTIAL COORDINATES Astronomers draw imaginary horizontal and vertical lines on the celestial sphere similar to the latitude and longitude lines on Earth. And use celestial coordinates to specify directions to sky objects. The celestial equator is the projection of the Earth’s equator out to the sky. Angular distance above or below the celestial equator is called declination (dec). Distance measured eastward along the celestial equator from the zero point, the vernal equinox, is called right ascension (RA). Just as any city on Earth can be located by its coordinates of longitude and latitude, any sky object can be located on the celestial sphere by its coordinates of right ascension and declination. THE ECLIPTIC Sun apparently move eastward during the year. The ecliptic, the apparent path of the Sun against the background stars, is drawn on sky globes and star maps for reference. The band about 16° wide around the sky that is centered on the ecliptic is called the zodiac. Ancient astrologers divided the zodiac into 12 constellations, or signs, each taken to extend 30° of longitude EARTH’S SEASONS The altitude of the Sun above the horizon at noon varies during the year because Earth’s axis is tilted to the plane of its orbit around the Sun. Earth’s equator remains tilted at about 23.5° to its orbital plane all year long. So as Earth travels around the Sun, the slant of the Earth–Sun line changes. Sunlight pours down to Earth from different angles during the year, causing the change of seasons as well as seasonal variations in the length of days and nights. Equinox is the time when sun crosses earth’s equator. THE DAY Earth’s rotation provides a basis for keeping time using astronomical observations. The solar day of everyday affairs measures the time interval of Earth’s rotation using the Sun for reference. The sidereal day measures the time interval of Earth’s rotation using the stars for reference. A sidereal day is 23 hours, 56 minutes, 4 seconds long and. solar day is 24 hours long. A solar day is about 4 minutes longer than a sidereal day because while Earth rotates on its axis it also moves along in its orbit around the Sun. Earth must complete slightly more than one whole turn in space before the Sun reappears on your meridian. THANK YOU