Seeing the Daytime Moon
By Deborah Byrd and Larry Sessions
EarthSky Communications
Friday, Jan. 9, 2009

This photo by Dan Bush shows the type of moon you might see on Friday afternoon – Jan. 9, – when the moon is waxing toward full and visible in the southeastern sky in late afternoon.

People are sometimes surprised on first catching sight of the moon during daylight. Somehow it just doesn’t “seem” right. But the moon is out during the day as often as it is at night. You just might not notice the moon in the daytime as often because, being far less bright than the sun, the moon doesn’t show up as starkly against the blue daytime sky as it does against the black of night.

In general, crescent moons several days on either side of the new moon phase are very difficult to impossible to see due to the glare of the sun nearby. First quarter moons rise at approximately noon and are well placed for viewing in the southern to southwest sky by late afternoon, slipping farther eastward each day until they rise with the setting sun (full moon). The last quarter moon is in the south at sunrise, whereas the waning crescent moon is in the southeast sky.

If you’re curious about the moon’s whereabouts, don’t miss our article on understanding moon phases at www.earthsky.org/article/understandingmoonphases.

What Kind of Clock is the Most Accurate?

An atomic clock keeps really accurate time. How is it different from a regular grandfather clock?

Listen to the tick-tock of your grandfather clock. The steady swing of the pendulum helps the clock keep time. But the rate of a pendulum’s swing can vary. A pendulum can expand or shrink with changes in temperature and humidity. The atomic clock, based of the “tick” of atoms, is exquisitely precise.

An atomic clock works much the same way as your grandfather clock. Atoms have resonances, or vibrations, and each element on the periodic table vibrates at a discrete frequency. So instead of the pendulum swinging back and forth, imagine the back-and-forth vibration of a microscopic atom – an atom of cesium for instance. Because the atom vibrates at a steady rate, it can keep track of time much the same way that a grandfather clock pendulum does – but far more accurately, and environmental changes don’t change the rate of an atom’s vibration.

Cesium is the element currently used in atomic clocks. The unit of time we know as a “second” was defined in 1967 using the cesium atomic clock. Cesium atoms vibrate 9,192,770,000 times in one second.

The first atomic clock, introduced in 1949, was far, far more accurate than any previously built timepiece. But the accuracy of successive models has been improving at better than a factor of ten every ten years. That means that our clock today is a million times more accurate than the first model.