Transcript for February 7 - 13

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

RACHEL
...and Rachel.

PAUL
The moon is new on the 8th.

RACHEL
So it reappears in the west two days later.

PAUL
Starting the 10th and for the next couple of nights, look for earthshine illuminating the dark portion of the moon.

RACHEL
And remember, earthshine is sunlight, but reflected by Earth.

PAUL
How is it possible for Earth to reflect so much towards the moon?

RACHEL
It’s because of Earth’s albedo and size.

PAUL
Explain albedo.

RACHEL
Sure, albedo is the astronomical term for reflectivity.

PAUL
And an albedo of zero is a perfect absorber of light and totally non-reflective.

RACHEL
While an albedo of 1 is perfectly reflective.

PAUL
So where does the moon’s albedo lie?

RACHEL
The moon’s albedo is 0.12

PAUL
Which is another way of saying the moon only reflects 12% of the sunlight incident on it.

RACHEL
That make the moon’s surface about as dark as an asphalt parking lot.

PAUL
And that’s on average.

RACHEL
Right, the lunar seas are darker and less reflective while the lunar highlands are just the opposite.

PAUL
So what about the Earth?

RACHEL
Earth’s albedo is 0.30.

PAUL
Most of that comes from its clouds and ice caps.

RACHEL
That’s correct; the oceans are darker.

PAUL
On average then, Earth is 2.5 times brighter than the moon.

RACHEL
So we can see that Earth is more reflective than the moon, what about their relative sizes?

PAUL
The moon’s diameter is 2,100 miles.

RACHEL
That small?

PAUL
Yep. The United States would wrap 50% of the way around the moon.

RACHEL
And I remember Earth is just about 7,900 miles in diameter.

PAUL
Which means Earth has a diameter nearly four times larger than the moon.

RACHEL
And that means about 14 times more surface area to reflect sunlight.

PAUL
Combining the 14 times larger surface area and 2.5 times more reflective surface...

RACHEL
...you get an Earth that appears about 35 times brighter in the lunar sky.

PAUL
That would be one big and bright blue and white orb.

RACHEL
And unlike the moon on Earth, to an astronaut standing on the moon, Earth would remain in the same position in the sky.

PAUL
However, the phase of Earth would change over a period of 30 days.

RACHEL
In fact, the phase of Earth would be opposite of the moon’s phase on Earth.

PAUL
So when the moon is new to us, Earth is in the full phase to the moon.

RACHEL
Best yet is during the New Earth phase.

PAUL
Some times at new, Earth will pass between the moon and the sun to create an amazing total solar eclipse.

RACHEL
Because of Earth’s atmosphere, lunar inhabitants would see all of the world’s sunsets as a ring of orangey-red fire surrounding Earth.

PAUL
And the lunar surface would be illuminated in that orangey-red glow.

RACHEL
That’s Idaho Skies for the second week of February. Next week we’ll tell you how to watch the occultation of Aldebaran.

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

For Idaho Skies this is Paul...

RACHEL
...and Rachel.

PAUL
Dark skies and bright stars.