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The Sky

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The Sky

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  1. The Sky

  2. 0 Constellations In ancient times, constellations only referred to the brightest stars that appeared to form groups.

  3. 0 Constellations (2) They were believed to represent great heroes and mythological figures. Their position in the sky seemed to tell stories that were handed down from generation to generation over thousands of years.

  4. Constellations (3) Today, constellations are well-defined regions on the sky, irrespective of the presence or absence of bright stars in those regions.

  5. Asterisms An asterism is part of a constellation or part of several constellations. The Big Dipper is part of Ursa Major The summer triangle is part of Lyra, Cygnus, and Aquila The great square is part of Pegasus and Andromeda

  6. Constellations (4) The stars of a constellation only appear to be close to one another Usually, this is only a projection effect. The stars of a constellation may be located at very different distances from us.

  7. 0 Constellations (5) Some examples of easily recognizable constellations and their brightest stars

  8. Constellations (6) Stars are named by a Greek letter (a, b, g) according to their relative brightness within a given constellation + the possessive form of the name of the constellation: Orion Betelgeuse = a OrionisRigel = b Orionis Betelgeuze Rigel

  9. Star Names Stars are named by a Greek letter (a, b, g) and Latin Constellation name. The naked eye visible stars have ancient names, usually Arabic. Stars discovered today are given Modern Star Numbers. If you discover a star, you do NOT get to name it.

  10. The Apparent Magnitude Scale • First introduced by Hipparchus (160 - 127 B.C.): • Brightest stars: ~1st magnitude • Faintest stars (unaided eye): 6th magnitude • More quantitative: • 1st mag. stars appear 100 times brighter than 6th mag. stars • 1 mag. difference gives a factor of 2.512 in apparent brightness (larger magnitude => fainter object!)

  11. The Apparent Magnitude Scale The magnitude scale system can be extended towards negative numbers (very bright) and numbers greater than 6 (faint objects): Sirius (brightest star in the sky): mv = -1.42Full moon: mv = -12.5Sun: mv = -26.5

  12. 0 The Apparent Magnitude Scale Apparent magnitude is not the only type. Consider the stars Sirius and the Sun. Sirius puts out much more energy than the Sun. So how is it that the Sun is so much brighter? Of course it is because the Sun is so much closer to us. It is, therefore, APPARENTLY brighter. If the 2 stars were at the same distance from us, Sirius would appear brighter.

  13. 0 The Absolute Magnitude Scale The ABSOLUTE magnitude scale is based on all stars being 10 parsecs away. (Obviously we can’t physically move the stars, but we compare them as if they were 10 parsecs away.) In this case, you can see that Polaris is the brightest of the three stars and the sun the dimmest.

  14. The Absolute Magnitude Scale At 10 pc

  15. Apparent and absolute magnitudes of common stars Object m M (apparent) (absolute) Sun -26.8 4.83 Sirius -1.47 1.41 Vega 0.04 0.5 Betelgeuse 0.41 -5.6 Polaris 1.99 -3.2

  16. The Celestial Sphere • Not real, but a useful concept. • A sphere surrounding Earth with the stars stuck to it. Everything revolving around Earth.

  17. Celestial Sphere • Imagine a person on Earth. • What can they see? • Where you stand gives you different views. • You can only see half of the celestial sphere at any one time from any place on Earth. • What you see depends on time of day/night, location on Earth, time of year.

  18. The Sun and Its Motions Due to Earth’s revolution around the sun, the sun appears to move through the zodiacal constellations. The Sun’s apparent path on the sky is called the Ecliptic. Equivalent: The Ecliptic is the projection of Earth’s orbit onto the celestial sphere.

  19. Terms • Zenith – The point on the celestial sphere above the observer’s head. • Horizon – The great circle between the zenith and nadir. The circle on the celestial sphere cut by a plane tangent to the Earth at your feet. Everything in the sky above the horizon is visible, while everything below it is not. • Nadir - The point on the celestial sphere directly underneath the observer’s feet. (not visible!)

  20. The Celestial Sphere Celestial equator = projection of Earth’s equator onto the c. s. North celestial pole = projection of Earth’s north pole onto the c. s.

  21. Finding things in the sky Two systems: • Global • Right Ascension (like longitude) • Declination (like latitude) • Local • Altitude • Azimuth

  22. Celestial Coordinates • Declination: degrees north or south of celestial equator • Right ascension: measured in hours, minutes, and seconds eastward from position of Sun at vernal equinox

  23. Altitude and Azimuth Altitude = How high up off the horizon. Measured in degrees, from 0 to 90. Azimuth = Which direction along the horizon. Measured in degrees, from 0 to 360.

  24. Altitude and Azimuth

  25. Azimuth (and Bearing)

  26. Altitude

  27. How the sky appears to move At the North Pole At the Equator At Northern mid-latitudes

  28. At Northern mid-latitudes

  29. 0 Apparent Motion of The Celestial Sphere Some constellations around the Celestial North Pole never set. These are called “circumpolar”. The circle on the celestial sphere containing the circumpolar constellations is called the “circumpolar circle”.

  30. Apparent Motion of The Celestial Sphere Circumpolar Stars

  31. The View From Northern Mid-Latitudes

  32. Apparent Motion of The Celestial Sphere (2)

  33. 0 North vs. South

  34. How the sky appears to move Latitude and Altitude of Polaris At the North Pole Latitude at North Pole = 90° N At the Equator Latitude at Equator = 0° Latitude here = 34° N At Northern mid-latitudes Altitude of Polaris = 0° Altitude of Polaris = 34° Altitude of Polaris = 90°

  35. 0 The Celestial Sphere (3)

  36. The Seasons Earth’s axis of rotation is inclined vs. the normal to its orbital plane by 23.5°, which causes the seasons.

  37. Why is it warmer in summer? • Two reasons • More direct sunlight • Longer days

  38. More Direct Sunlight

  39. Tilt Gives Us Seasons

  40. Longer Days in Summer

  41. 0 The Seasons The Seasons are only caused by a varying angle of incidence of the sun’s rays. We receive more energy from the sun when it is shining onto the Earth’s surface under a steeper angle of incidence.

  42. NOT DUE TO DISTANCE Earth’s distance from the sun has only a very minor influence on seasonal temperature variations. Earth’s orbit (eccentricity greatly exaggerated) Earth in January Earth in July Sun

  43. Obliquity of the Planets

  44. 0 The Motion of the Planets The planets are orbiting the sun almost exactly in the plane of the Ecliptic. Jupiter Venus Mars Earth Mercury Saturn The Moon is orbiting Earth in almost the same plane (Ecliptic).

  45. The Inferior Planets P4 = Greatest Eastern Elongation P2 = Greatest Western Elongation Sunrise Sunset

  46. The Superior Planets P1 = Greatest Elongation Also Opposition! Overhead at midnight