The Bowl of Night

| Basic Celestial Phenomena | Constellations Index |

• At night the sky appears like a giant dome overhead, or an upside-down bowl set upon the horizon as if on a table.
  
• The bowl of night is studded with the light of thousands of stars, of varying apparent magnitude.
  • Some stars always retain the same spatial orientation with respect to each other; these are the fixed stars.
  • The fixed stars are distributed among 88 constellations (such as Ursa Major the Big Bear).
  • To the imagination, many star patterns other than the official constellations are discernible; these patterns are called asterisms (such as the Big Dipper and the Winter Hexagon).
    
• The rim of the bowl of night is the horizon, or azimuth circle.
  Azimuth = measured along the horizon, in degrees.
  • By convention, azimuth is measured clockwise from due north.

    Direction	Azimuth
    North	0 degrees
    East	90 degrees
    South	180 degrees
    West	270 degrees

      North-East-South-West = Never eat slimy worms.

  • Find north by using the Big Dipper to locate Polaris, the north star. Polaris is closer to true north than a magnetic compass.
  • Note: In Japan, azimuth is measured clockwise starting from the south.
    
• The point directly overhead is called an observer's zenith. Opposite the zenith is the nadir, directly beneath one's feet.
  • Are zenith and nadir points horizon-dependent? That is, do they differ for observers at different locations?
  • Are zenith and nadir points time-dependent? That is, do they differ for an observer at the same location but at different times?
  • Is it meaningful to speak of the azimuth of a star at the observer's zenith?
    
• A line (arc) from the point due north on the horizon (0 degrees) passing through the zenith and intersecting the horizon due south (180 degrees) is called the meridian.
  • Polaris always lies on or near the meridian. What is the azimuth of Polaris as seen from Shawnee?
    
• Altitude = measured above the horizon in degrees.
  • What is the altitude of Polaris as seen from Shawnee?
  • What is the altitude of a star at the observer's zenith?
  • Is it meaningful to speak of altitudes greater than 90 degrees?
  • The maximum altitude, 90 degrees, is the zenith. (Zenith is a great name for a TV: Since dust and horizon haze obscure the sky at lower altitudes, when you look toward the zenith you get a "clearer picture.")
  • Note that altitude in this sense is measured in angular degrees, and has nothing to do with height above the ground or elevation above sea level.
    
•  Altitude-Azimuth coordinates uniquely specify a given point with respect to an observer's horizon at an indicated time.
  • Do any two different locations in the sky have the same pair of altitude-azimuth coordinates?
  • Use a protractor outdoors to estimate the altitude of Polaris, or to measure degrees of azimuth along the horizon from due north.
  • Horizon coordinates vary with locality, but are still useful in skywatching and are used with many telescope mounts.
    
• Any star or planet located on the meridian is said to be at meridian transit or culmination.
  • What is the azimuth of a planet at meridian transit?
  • Trick question: When might a meridian transit not be the same as culmination?
    • Answer: Circumpolar stars cross the meridian twice each day without setting below the horizon. Only the higher transit of the two would be a culmination.
  • When should I look for a planet or a constellation?
    • How can I tell whether a constellation is ever visible from my location? (Click to go to the page with the answer)
    • When a planet or star is transiting the meridian it is at its highest in the sky and therefore its farthest from dust and horizon haze. The best time to see it, then, would be around its midnight culmination.
    • When a planet or star is transiting the meridian at midnight it is opposite the sun--it would be noon on the opposite side of the earth. If it is high enough, a planet or star at "opposition" may be visible all night long:
      • Rises in the east when the sun sets in the west.
      • Transits the meridian at midnight.
      • Sets in the west when the sun rises in the east.
      • Consider: What is the phase of the moon when it is in opposition?
    • Where should I look for a planet or star after its meridian transit?
      • If its altitude on the meridian is high enough, a given constellation may be visible in the early evening sky for two or three months after the date of its midnight culmination.
      • After the date of its midnight culmination it will transit the meridian at an ever earlier time each evening, until eventually it will already be in the western sky when it first appears at sunset.
      • Finally, it will set with the sun and enter the daylight sky.
    • List of constellation midnight culminations
      • Try to identify the constellations listed for each month of the year!
      • Before midnight, look for each month's constellations rising higher in the eastern sky.
      • After midnight, look for them setting in the western sky.
      • Before midnight, look for the previous month's constellations setting in the western sky.
      • The most southerly constellations may only be visible at their midnight culmination, if they never get very high in the sky.
      • Use a planisphere to locate any constellation visible at any time of night on any day of the year.

Next: Go to diurnal motion, or non-horizon coordinates of the celestial sphere.