Big Dipper - Guidepost

Guidepost

Not only are the stars in the Big Dipper easily found themselves, they may also be used as guides to yet other stars. Thus it is often the starting point for introducing Northern Hemisphere beginners to the night sky:

• Polaris, the North Star, is found by imagining a line from Merak (β) to Dubhe (α) and then extending it for five times the distance between the two Pointers.
• Extending a line from Megrez (δ) to Phecda (γ), on the inside of the bowl, leads to Regulus (α Leonis) and Alphard (α Hydrae). A mnemonic for this is "A hole in the bowl will leak on Leo."
• Crossing the top of the bowl from Megrez (δ) to Dubhe (α) takes one in the direction of Capella (α Aurigae). A mnemonic for this is "Cap to Capella."
• Castor (α Geminorum) is reached by imagining a diagonal line from Megrez (δ) to Merak (β) and then extending it for approximately five times that distance.
• By following the curve of the handle from Alioth (ε) to Mizar (ζ) to Alkaid (η), one reaches Arcturus (α Boötis) and Spica (α Virginis). A mnemonic for this is "Arc to Arcturus then speed (or spike) to Spica."

Additionally, the Dipper may be used as a guide to telescopic objects:

• The approximate location of the Hubble Deep Field can be found by following a line from Phecda (γ) to Megrez (δ) and continuing on for the same distance again.
• Crossing the bowl diagonally from Phecda (γ) to Dubhe (α) and proceeding onward for a similar stretch leads to the bright galaxy pair M81 and M82.
• Two spectacular spiral galaxies flank Alkaid (η), the Pinwheel (M101). to the north, and the Whirlpool (M51), to the south.
• Projecting a line from Alkaid through the pole star will point to Cassiopeia and hence the cluster known as the Seven Sisters or Pleiades.