Transcript for January 10 - 16

PAUL
Welcome to Idaho Skies for the second week of January. We’re your hosts, Paul...

RACHEL
...and Rachel.

PAUL
The moon was new on the 9th.

RACHEL
A new moon means the moon is passing between Earth and the sun.

PAUL
That happens every 29 or 30 days, or once every lunar orbit.

RACHEL
The moon is dark at new moon, but not because the "dark side" of the moon is turned toward us.

PAUL
In fact, we always see the same face of the moon.

RACHEL
The reason the moon is dark at new is that the portion of the moon in night also happens to be the half facing Earth.

PAUL
We only see one half of the moon because its period of rotation, which is also the length of its day, is the same as its orbit around Earth.

RACHEL
The same thing would happen between Earth and the sun if our day were 365 days long rather than 24 hours long.

PAUL
After its birth, the moon probably rotated much faster on its axis.

RACHEL
However, because of it closeness to us, Earth’s gravity slowed down the moon’s rotation until now it rotates once every revolution around Earth.

PAUL
Astronomers call the moon tidally locked with Earth.

RACHEL
This is the current situation between Pluto and its largest moon, Chiron.

PAUL
And probably with some of the hot Jupiter exoplanets that astronomers have discovered outside our solar system.

RACHEL
In time, the moon may be able to slow down Earth’s spin until both are tidally locked together.

PAUL
Then only one half of Earth will be able to see half of the moon.

RACHEL
Sadly, because of the tilt of the moon’s orbit around Earth, the new moon passed above the sun from Earth’s perspective.

PAUL
Robbing Idaho of its chance to see a solar eclipse this month.

RACHEL
The moon reappears low in the south-southwest on the evening of the 11th.

PAUL
The moon will be two days old then, so it will be very thin.

RACHEL
In another day or two, you should be able to see earthshine on the dark portion of the moon.

PAUL
Binoculars give a better view of earthshine than the eye alone.

RACHEL
Keep observing the moon through binoculars, as it will look pretty neat for the next four or five days.

PAUL
That’s because you’ll see lots of craters and mountains along the lunar terminator.

RACHEL
The terminator is the moon’s boundary between day and night.

PAUL
At the terminator, shadows bring out the greatest lunar detail.

RACHEL
The view is even better when the moon’s seven days old and the terminator faces directly towards Earth

PAUL
That makes the lunar shadows appear especially long.

RACHEL
And that extended length readily emphasizes small changes in the lunar terrain.

PAUL
Because of the moon’s small size and slow rotation, the terminator travels around the moon at just under 10 miles per hour.

RACHEL
Therefore, it would be easy for a runner to keep up with the movement of the terminator.

PAUL
That’s Idaho Skies for the second week of January. Next week, Idahoan’s get to observe a lunar occultation.

 RACHEL
Be sure to read our blog for additional information. It’s at idahoskies.blogspot.com.

For Idaho Skies this is Rachel...

PAUL
...and Paul.

RACHEL
Dark skies and bright stars.