PHYS 100 Astronomy Chapter 2

PHYS 100 - ASTRONOMY CHAPTER 2 GUIDEPOSTS 2-1 THE STARS How do astronomers refer to stars by name and brightness? Astronomers divide the sky into 88 constellations. Although the constellations origi...

PHYS 100 - ASTRONOMY CHAPTER 2 GUIDEPOSTS 2-1 THE STARS How do astronomers refer to stars by name and brightness? Astronomers divide the sky into 88 constellations. Although the constellations originated in Greek and Middle Eastern mythology, the names are Latin. Even the modern constellations, added to fill in the spaces between the ancient figures, have Latin names. A constellation is a group of stars with defined borders as set by the “International Astronomical Union (IAU)” in 1928. The names of stars usually come from ancient Arabic, although modern astronomers often refer to a star by its constellation and a Greek letter assigned according to its brightness within the constellation. 2-2 THE SKY AND ITS MOTION How does the sky move as Earth moves? The horizon is the circle where the sky and earth appear to meet. The zenith is the point on the sky overhead, and the nadir is the point directly below. The celestial sphere is a scientific model of the sky, carrying the celestial objects around Earth. Because Earth rotates eastward, the celestial sphere appears to rotate westward on its axis. The northern and southern celestial poles are the pivots on which the sky appears to rotate. The celestial equator, an imaginary line around the sky above Earth’s equator, divides the sky in half. The motion of the sky defines directions on Earth as the north, south, east and west points on the horizon. Astronomers often refer to angles “on” the sky as if the stars, sun, moon, and planets were equivalent to spots painted on a plaster ceiling. These angular distances are measured in degrees, minutes of arc, and seconds of arc, and are unrelated to the true distance between the objects in light-years. Angular diameter is the angular distance from one edge to the other across an object. What you see of the celestial sphere depends on your latitude. Much of the southern hemisphere of the sky is not visible from northern latitudes. To see that part of the sky, you would have to travel southward over Earth’s surface. A circumpolar constellation is one that never sets or never rises. The angular distance from the horizon to the north celestial pole always equals your latitude. This is the basis for celestial navigation. The gravitational forces of the moon and sun act on the spinning Earth and cause precession. Earth’s axis of rotation sweeps around in a conical motion with a period of 26,000 years, and consequently the celestial poles and celestial equator move slowly against the background of the stars. 2-3 THE CYCLES OF THE SUN How does the sky affect Earth? Rotation refers to the turning of Earth on its axis, but revolution refers to Earths orbital motion around the sun. Because Earth orbits the sun, the sun appears to move eastward along the ecliptic through the constellations. It circles the sky in a year. Because the ecliptic is tipped 23.5° to the celestial equator, the sun spends half the year in the northern celestial hemisphere and half in the southern celestial hemisphere. In the summer, the sun is above the horizon longer and shines more directly down on the ground. Both effects cause warmer weather in the northern hemisphere. The beginning of spring, summer, winter, and fall are marked by the vernal (spring) equinox, the summer solstice, the autumnal equinox, and the winter solstice. The seasons are reversed in the southern hemisphere. Earth is slightly closer to the sun at perihelion in January and slightly farther away at aphelion in July. Mercury and Venus follow orbits inside Earth’s orbit and never move far from the sun. They are sometimes visible in the east before dawn or in the west after sunset. Any planet visible in the sky at sunrise is called a morning star. If it is visible in the sky at sunset, it is an evening star. In addition to the sun, the visible planets also move along the ecliptic and their positions along the zodiac are plotted in horoscopes. This gave rise to the ancient superstition called astrology. Astrology is pseudoscience, which means that it is founded on belief rather than evidence. 2-4 ASTRONOMICAL INFLUENCES ON EARTH’S CLIMATE According to the Milankovitch hypothesis, changes in the shape of Earth’s orbit, in its precession, and in its axial tilt can alter the planet’s heat balance and are at least partly responsible for the ice ages and glacial periods.