On Sept. 23, 2011, the sun's rays were directly over the Earth's equator.
When this occurs in fall, it is called the autumnal equinox, and we have days and nights of about equal length. The above diagram illustrates this. This time of year, longer nights give an excellent chance to observe the planet Venus in the night sky. Venus, also called the morning or evening "star," is the brightest natural object in the night sky.
Often Venus can be seen with a fainter nearby planet, Mercury. One can consult a newspaper or the Web to find the date and direction to observe this pair, as is seen in the right diagram from spring 2011.
OK, so what is the connection here with climate science?
Ground-based telescopes and two space probes have given us some data on both of these planets. Mercury, the closest planet to the sun, was found to have no significant atmosphere and to have a surface temperature that ranged from about 152 to 800 degrees Fahrenheit depending on location and time of day. Sun rays warm the planet by day, and infrared radiation carries it away at night.
Venus, by comparison, is about twice as far from the sun as Mercury and has a very dense atmosphere. This atmosphere acts to keep the surface temperature more uniform, at a toasty 890 degrees.
We have a planet that is many tens of millions of miles farther from the sun than Mercury, yet it has a higher surface temperature that is warm enough to melt lead. So what is going on?
While the geology and chemistry of the surface of Venus may be complex, the atmosphere is relatively simple. It is more than 96.5 percent carbon dioxide. Venus has no carbonate rock, like in the Champlain Valley, or liquid water, that can store the carbon dioxide. It is thought that almost all of the carbon on the planet is present in the atmosphere as carbon dioxide. Venus has a pronounced "runaway greenhouse" effect. This is the same greenhouse gas, the rising levels of which concerns climate scientists today.