ASTR 101 The Earth and the Sky August 29, 2018 • Sky and the atmosphere • Twinkling of stars • The celestial sphere • Constellations • Star brightness and the magnitude system • Naming of stars Scattering of light and color of the sky Sun light more less scattering scattering air particle • During the daytime we cannot see stars due to the glare in the atmosphere (from sunlight). • When sunlight travels through the Earth’s atmosphere some of the light is scattered (deflected) off air molecules. • White light is a combination of different colors, blue light is scattered far more than red light. Blue Sky during the day Sun Earth atmosphere • During the day we see this scattered sunlight in the atmosphere. • Since most of it is blue, a lot of blue light is seen in the atmosphere in all directions, which makes the sky look blue. • Light from stars are fainter than scattered sunlight in the atmosphere, so we cannot see them (unless a star happen to be very bright) In the outer space sky is always dark • Away from the atmosphere (in outer space, on moon…) sky is always dark. • It is possible to see stars and the sun at the same time. The Sun looks red at Sunrise/Sunset Sun atmosphere Earth • At sunrise and sunset, Sun is close to the horizon. Sunlight has to travel a longer distance in the atmosphere to reach us. • By the time sunlight reaches ground most of the blue light has scattered off. Only red light remains. • So the Sun (moon…) looks red at sunrise or sunset. • If the sky is cloudy, smoggy or there is a lot of dust particles in air they may reflect the sunlight and that part of the sky also looks red. Twinkling of Stars (stellar scintillation) • When starlight travels through the atmosphere light refracts slightly (changes direction) as it passes through air layers of different densities. • When the atmosphere is turbulent this refraction is not uniform or steady, it changes from moment to moment, changing the direction of starlight all the time. • Since angular size of stars are extremely small (less than 0.01”) even a tiny change in the direction is noticeable, resulting “twinkling” of stars. • When a star is low in the sky, light has to travel a longer distance through the atmosphere. – Stars closer to the horizon appear to twinkle more than stars that are overhead Twinkling View of a star through a telescope in poor seeing conditions • Twinkling is undesirable for astronomy. For astronomical observations, a steady atmosphere with good “seeing” is needed. • Major Astronomical observatories are located in higher elevations (mountain tops). Twinkling • Planets have larger angular sizes. • So a very small changes in the position of a planet in the sky is not so noticeable, they twinkle less. – This is one way to distinguish a planet from a star. • But in poor seeing conditions planets also twinkle, especially when they are low in the sky. The Earth and the Sky • From a location on ground, the Earth looks flat – because we see only a tiny part of the vast spherical surface of the Earth • At a given time and location we see half of the sky, the sky directly above that location. • View of the other half is blocked by earth. Earth spins on its axis (west to east) and goes around the sun Celestial objects (planets, stars, galaxies…) are located at different distances Just as someone in a boat sailing forward sees unmoving [objects] going backward , on the earth, sees the unmoving stars going uniformly westward… -Aryabhata, ca 500CE https://en.wikipedia.org/wiki/Aryabhata • On earth we do not feel its motion or rotation. • Instead it appears that the whole universe (sky), sun, moon, stars… goes around the Earth east to west. • All celestial objects appears to be located at the same distance, like located inside a giant sphere centered at Earth. Just as someone in a boat sailing forward sees unmoving [objects] going backward , on the earth, sees the unmoving stars going uniformly westward… -Aryabhata, ca 500CE https://en.wikipedia.org/wiki/Aryabhata • But on earth we do not feel its motion or rotation. • Instead it appears that the whole universe, sun, moon, stars… goes around the Earth east to west. • All celestial objects appear to be located at the same distance, like they are on a giant sphere centered at Earth. The Celestial Sphere www.phy.olemiss.edu/~perera/animation_cele/celes_sp.gif • This apparent sphere around us with all celestial objects on it, is called the Celestial Sphere. • We know this is an illusion: – different celestial objects are at different distances from us. – it is the earth that rotates, not the sky • Nevertheless considering sky as a sphere centered at earth is a useful concept for observational purposes. The Earth and the Sky zenith N W E S nadir • At a given location and time only half of the celestial sphere (sky) is visible, view of the other half is blocked by the earth. • On a typical moonless clear night, about 3000 stars are visible from a dark location. – Zenith: the point directly above in the sky at a given time and location. – Nadir: Point in the sky directly below. It cannot be seen because it is on the other side of the earth. – Celestial Meridian: The imaginary circle passing through the North and South points on the horizon and through the zenith. – Celestial object has the highest elevation (altitude) when it cross the meridian. Daily Motion of the Sky transit N rising E setting celestial meridian W S • All celestial objects appears to rise in the East, moves across the sky and sets in the West. • They have the highest elevation above horizon (altitude) when crossing the meridian (also called transit) Celestial Poles and Equator North celestial pole Celestial equator South celestial pole • The Earth’s rotation axis extended to the sky (to the celestial sphere) meets the celestial sphere at two locations. – They are called celestial poles. • Celestial equator is the great circle on the celestial sphere in between celestial poles. – the imaginary circle in the sky directly above the Earth's equator. 16 + North Celestial pole Polaris – the North Star Long exposure image of the north pole region Star trails over the Kennon Observatory • Earth’s axis always points to the same point in the sky regardless the rotation or the orbital movement. • locations of celestial poles in the sky are fixed • the whole sky appears to rotate about celestial poles. • Since celestial poles don’t move, so do any objects located at the poles. • There happen to be a bright star very close to the north pole: The north star ( Polaris). • It appears to stay fixed in the sky while other stars move around it. Stars near the north celestial pole • There isn’t any bright star close to the South celestial pole. • So there is no “south star”. Stars near the Magellanic south celestial pole clouds zenith 90⁰ Sky Coordinates altitude N W 270⁰ E Azimuth 90⁰ S 180⁰ • Simplest way to give the location of a celestial object in the sky is to give its apparent position in the sky – Altitude: vertical angular elevation above the horizon – Azimuth: direction from North to the object: angle measured along the horizon towards the east along the horizon. • But the apparent position of a celestial object changes with time and location. – Not the best way to identify an object. Logitude and Latitude • Position of a place on the Earth can be given by its longitude and latitude. – Latitude: angular distance from the equator. – Longitude: angular separation of the meridian through the location and the reference meridian trough Greenwich. • A similar mechanism can be used to give the position of a celestial object on the celestial sphere (sky). N Right ascension and Declination celestial north pole Vernal Equinox (first point of Aries) declination celestial equator right ascension reference median • Position of an object on the celestial sphere can be specified similarly. – When refer to locations in the sky they are called declination and right ascension. • Declination: angular distance to an object from the celestial equator. • Right ascension: angle between the celestial meridian through an object and a reference median. Constellations • View of the southern sky this evening from Oxford MS. – Stars, Saturn, Milky way • Ancient observers imagined groups of brighter stars as patterns in the sky, called constellations. – Representing mythological characters, heroes, animals, royalty whatever important or fascinated them. • Every culture saw patterns in the sky and attributed legends and beliefs according to their culture. • Northern constellations in use now date back to Babylonian times, 4-5 millennia ago. Later Greeks adopted them and named according to their mythology. Western (Greek) Ancient Egyptian Ancient Chinese Constellations in the same part of the sky according to Greeks ,Egyptians and Chinese • From the planetarium software : Stellarium – Available from (free): www.stellarium.org • Also demo version of Skygazer planetarium software is available form http://www.carinasoft.com/downloads.html#VdemoAnchor Examples of few constellation figures • All above are Zodiac constellations – the narrow band in the sky where sun, moon and planets are visible. • Their origin dates backs to ancient Sumerians and Babylonian times Modern Constellations constellation boundaries • 16th-19th century astronomers added many new constellations to fill in the gaps between classical constellations and cover uncharted regions of the southern sky. • In 1922, the International Astronomical Union (IAU) adopted 88 constellations as internationally accepted constellations. – That included the 48 classical constellations, 32 southern constellations and few other constellations formed since the 16th century. – 88 constellations divide the sky in to 88 regions according to boundaries defined by the IAU.