By Mike P. Usher
Tonight we will look northwards again to the North Star, but if we were in ancient Egypt we would not be looking at Polaris, but the faint star Thuban (aka Alpha Draconis) instead. Because the Moon tugs at the bulge around the Earth’s equator our planets axis very slowly wobbles in a manor similar to the toy gyroscopes you played with as a kid. It’s a very slow wobble to be sure, taking some 25,772 years to trace out a circle in the sky. This movement is called precession of the equinoxes, as the other main effect is to move the points of the equinox around the ecliptic. Today our Earth’s axis points less than one-half degree from Polaris but in 2787 B.C. it was pointed even more closely at Thuban. It was still the pole star in 2540 B.C. when the Pyramid builders laid out the north-south lines of the Great Pyramid. Thuban is marked with a circle on this week’s sky chart.
About 1900 B.C. the axis of the Earth had moved enough that the much brighter star Kochab was considered the pole star. After 500 A.D. Polaris was considered the pole star and will make it’s closest approach to the true celestial pole in about 90 years. About 14,000 A.D. the even brighter star Vega will be considered the pole star, although it won’t come as close to the true celestial pole as Polaris is now.
Speaking of Vega, tonight there is a meteor shower known as the Lyrids named after the constellation Lyra of which Vega is the brightest star. Tonight, but especially after midnight, you can see meteors radiating from Lyra like spokes on an old fashioned wagon wheel. Meteor showers are not particularly well named – it’s more of an occasional drizzle than a shower. Tonight the Lyrids may do about 20 meteors an hour. The rising Moon will interfere with the visibility of the fainter meteors by 1:00AM. Lyra will not actually rise in the northeast until 11:00PM anyway so your viewing window is short. On the other hand you don’t have to wait until Lyra rises, some meteors will be visible before midnight and Lyra does not have to be above the horizon as you can still see the upper half of the “wagon wheel” of meteor trails.
See you next time!
Mr. Usher is President of the Everglades Astronomical Society which meets every second Tuesday at 7:00PM at the Norris Center, Cambier Park, Naples.