Transcript for July 19 - 25

RACHEL
Welcome to Idaho Skies for the third week of July. We’re your hosts, Rachel...

PAUL
...and Paul.

RACHEL
The moon’s path across the sky in July is very shallow.

PAUL
So the moon never appears very high above the horizon during summer nights.

RACHEL
That means we have to wait until the 20th before the moon gets high enough above the horizon to show Earthshine clearly.

PAUL
The bad news is that the moon will be almost five days old and Earthshine will therefore be weak.

RACHEL
In winter, the situation is reversed.

PAUL
That’s because the moon rises nearly vertically above the horizon.

RACHEL
And it travels higher above the horizon.

PAUL
So we get a clearer view of the moon when it’s younger and thinner.

RACHEL
A young and thin moon is fainter and so it doesn’t wash out the illumination of Earthshine.

PAUL
The moon appears near a bright, but lonely star on the evening of the 22nd.

RACHEL
The star is Spica and it’s the brightest star of Virgo the Maiden.

PAUL
Spica is a white star, indicating that it’s much hotter than our yellow sun.

RACHEL
Spica isn’t the only interesting star the moon passes this week.

PAUL
The next one is Zubenelgenubi.

RACHEL
Zubenelgenubi is a star that’s easy to split into a pair of stars using binoculars.

PAUL
You’ll see the star to the moon’s lower left on the night of the 24th.

RACHEL
Some people with excellent vision can see Zubenelgenubi as a double star without using binoculars.

PAUL
Are you one of those people?

RACHEL
Looking for Saturn?

PAUL
Then wait no later than the 25th when the moon parks next to the ringed jewel of our solar system.

RACHEL
Saturn will be the yellow-white star to the moon’s left that night.

PAUL
One way you’ll know that its Saturn is that it will hardly twinkle, if at all.

RACHEL
Stars twinkle because they have such small apparent disks.

PAUL
That lets individual warm and cold air pockets constantly pass between then and our eyes.

RACHEL
The changing density of these air pockets acts like shaky lens and prisms that bend and refract starlight many times per second.

PAUL
The result is twinkling stars that throw off shards of color.

RACHEL
Saturn on the other hand is thousands of times larger in apparent diameter than the more distant stars.

PAUL
And as a result, unless the air is very turbulent, there are many pockets of air moving over Saturn simultaneously.

RACHEL
Their effects average out leaving Saturn with a nearly constant brightness and color.

PAUL
That’s Idaho Skies for the third week of July. Next week the moon will help you find star clusters and nebulae with your binoculars. 

RACHEL
Be sure to follow us on Twitter at Idaho Skies for this week’s event reminders and sky maps.

For Idaho Skies this is Rachel...

PAUL
...and Paul.

RACHEL
Dark skies and bright stars.