The Bowl of Night
"He also made the stars." Genesis 1: 16
- 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
- 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-East-South-West = Never eat
- Find north by using the Big Dipper
to locate Polaris,
the north star. Polaris is closer to true north than a magnetic
- Note: In Japan, azimuth is measured clockwise starting from
- 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
- Is it meaningful to speak of the azimuth of a star at the
- 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
- 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
- Is it meaningful to speak of altitudes greater than 90
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- List of constellation midnight
- 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
- 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