What’s Up in the Sky – April, 2008
By Peter Burkey
This month marks the anniversary of an event not well known outside the astronomical community. On April 20, 1920, the Shapley-Curtis debate was held at the National Academy of Sciences in Washington, D.C. The topic was the nature of “spiral nebulae” which astronomers had observed since the 1840’s. On one side was Harlow Shapley from the Mount Wilson Observatory, who argued the spiral nebulae were relatively small objects located in our own galaxy. Opposing him was Heber D. Curtis of the University of California’s Lick Observatory and a proponent of the “island universe” theory that each nebula is a rotating star system similar to the Milky Way and located far beyond its boundaries.
The problem was neither side could produce conclusive evidence to determine the distances to the spiral nebulae. A young man then studying astronomy at the Yerkes Observatory near Chicago would finally make such a determination four years later. That young student was Edwin Hubble.
It may seem strange, but distances in astronomy are very difficult to measure. The problem is that in order to know how far away something is you have to know how bright it is really (its intrinsic brightness).
Assisted by Milton Humason, Hubble observed a certain type of pulsating star, called a Cepheid variable, whose intrinsic brightness is related to its rate of pulsation. Their measurements enabled them to calculate the star’s actual brightness - 10,000 time that of the sun! In order for it to appear as dim as it did in their photographs it had to be very far away - 2.5 million light years -well beyond the confines of the Milky Way. The Shapley-Curtis “debate” was settled.
Today astronomers still expend great energy and build amazing instruments to measure stellar and galactic distances with ever increasing accuracy. These data have unveiled mysteries of the universe long sought by humans. In addition to its size and structure, the history, evolution, and future of the universe can now be studied. All this is possible through the careful measurements of what’s up in the sky.
This month in history:
April 5: Pioneer 11 launched - 1973
April 11: Apollo 13 launched - 1970
April 12: Yuri Gagarin becomes first human in space - 1961
April 12: Columbia is first space shuttle to be launched - 1981
April 17: Apollo 13 returns to Earth - 1970
April 20: Shapley-Curtis debate - 1920
April 24: China is fifth nation to launch satellite - 1970
Here are this month’s viewing highlights:
Planets this month: After sunset Saturn is in SE, left (east) of the star Regulus and Mars is in the SW, just below the twins Castor and Pollux. Mercury becomes visible near western horizon at month’s end. Jupiter dominates predawn southern sky.
April 4: Thin crescent Moon near Venus just before sunrise - challenging.
April 5: New Moon.
April 8: Watch crescent Moon pass in front of Plieades - spectacular.
April 12: First quarter Moon.
April 20: Full Moon.
April 28: Last quarter Moon.
