Topic 6

• What is an asterism?

  A group of bright and easily recognisable stars
• What are the cardinal points?

  North, South, East, West
• What is the celestial equator?

  The Earth’s equator projected vertically into the celestial sphere
• What are the celestial poles?

  The North and South Poles of the Earth projected vertically into the celestial sphere
• What is the celestial sphere?


  An imaginary sphere surrounding the Earth with all the stars on it
• What does circumpolar mean?

  A star that does not set during the day/night
• How can you determine if a star is circumpolar?

  A star is circumpolar if $90 - \text{latitude} < \text{declination of the star}$
• What is a constellation?


  An area of sky containing groups of stars considered to make an image
• What is the equatorial coordinate system?


  • Fixed to the stars
  • Composed of right ascension (RA) and declination
  • Declination measured from the celestial equator (-90° to +90°)
  • RA measured from the first point of Aries (vernal equinox) (0h-24h)
• What is the horizontal coordinate system?

  • Fixed to the observer
  • Composed of altitude and azimuth
  • Altitude: height of a star above the horizon (0-90°)
  • Azimuth: distance around the horizon from North to the star (0-360°)
• What is the hour angle?


  The time since a star last culminated
• What is local sidereal time?

  Time based on the location of stars in the sky, rather than the Sun
• How is local sidereal time calculated?

  Measured from the vernal equinox, LST = RA of observer’s meridian
• What is a meridian in astronomy?


  An imaginary line joining the North and South celestial poles and the observer’s zenith
• What is a pointer in astronomy?

  Using an easily locatable night sky object to locate another object
• What is upper transit/culmination?


  The highest point in a star's journey through the sky that day/night
• What is the zenith?


  The point in the celestial sphere directly above the observer
• What is the Celestial Sphere?

  An imaginary sphere surrounding the Earth on which all celestial objects are placed.
• What is the Ecliptic?

  The path which the Sun is observed to take through the celestial sphere in one year.
• Why is the Ecliptic inclined at 23.5° to the celestial Equator?

  Due to the axial tilt of the Earth.
• How many constellations does the Ecliptic pass through?

  12 constellations of the Zodiac and Ophiuchus, a 13th constellation.
• What are Horizon Coordinates based on?

  They are based on the observer's location and the time of the observation.
• What does Altitude measure in Horizon Coordinates?

  The angle of the object measured upwards from the observable horizon.
• What is the range of Altitude?

  From 0° (the horizon) to 90° (the observer’s zenith).
• How should an altitude greater than 90° be measured?

  It should be measured from the opposite horizon (less than 90°).
• What does Azimuth represent?

  The bearing of an object taken from true north moving round in an easterly direction.
• What is the range of Azimuth?

  From 0° (N) to 360°.
• What is an asterism?

  An asterism is the most recognisable part of one or more constellations.
• What is the Meridian?

  An observer-based line that runs from due north on the horizon, up through the zenith, and down to the horizon due south.
• Where does the North Celestial Pole (NCP) lie?

  It always lies on the meridian and its altitude is equal to the observer’s latitude.
• What is an example of an asterism in the constellation Orion?

  Orion’s belt is a small part of the constellation Orion.
• What is the relationship between the altitude of the NCP and the observer's latitude?

  The altitude of the NCP equals the observer’s latitude.
• What is Diurnal Motion?

  The apparent motion of an object due to the daily rotation of the Earth.
• Which constellation do the seven stars of the Plough belong to?

  The seven stars of the Plough are part of the constellation Ursa Major.
• How do stars appear to move when looking North?

  They appear to rotate anticlockwise about the NCP with a period of 23h 56m.

  
• What defines a circumpolar star?

  A star that does not set below the observer’s horizon.
• What is the condition for a star to be circumpolar?

  A star is circumpolar if its declination is greater than 90° minus the observer’s latitude.
• How many stars of the square of Pegasus are in the constellation Pegasus?

  Three stars of the square of Pegasus are in Pegasus, while one is in Andromeda.
• What is the formula for the altitude of a star at upper/lower transit?

  Altitude = observer’s latitude ± (90° – star's declination).
• What stars make up the Summer Triangle asterism?

  The Summer Triangle consists of the stars Deneb, Vega, and Altair.