Transcript June 14th to 20th

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

PAUL
...and Paul.

RACHEL
It’s amazing what you can do in astronomy using just a stick and a shadow.

PAUL
Yep and here’s one example.

RACHEL
On June 19th, 240 BC, Greek scientist Eratosthenes made the first successful measurement of Earth’s circumference.

PAUL
You’ve probably heard of Eratosthenes, he was the chief librarian in Alexandria, Egypt.

RACHEL
At the library, he learned that at noon on the first day of summer, objects cast no shadow in the town of Syene.

PAUL
Syene is an ancient name for the modern town of Aswan.

RACHEL
...and it’s located south of Alexandria where Eratosthenes lived.

PAUL
Eratosthenes observed that sticks did cast shadows in Alexandria on the same day.

RACHEL
Because sticks cast shadows in Alexandria but not in Syene, Eratosthenes reasoned that Earth must be spherical.

PAUL
So he compared the amount of shadow cast in Alexandria to the distance between Alexandria and Syene.

RACHEL
The amount of shadow indicated the proportion of Earth’s circumference covered by the distance between these two towns.

PAUL
Then he compared the circumference of a complete circle to the portion of Earth’s circumference between Alexandria and Syene.

RACHEL
Based on the ratios, Eratosthenes determined Earth’s circumference to within 16%.

PAUL
So much for the claim that ancients thought the world was flat.

RACHEL
Because the sun casts no shadow at noon on the solstice, Syene must lie on a Tropic.

PAUL
The Tropics are two circles of latitude that are 23.5 degrees north and south of the equator.

RACHEL
Why 23.5 degrees?

PAUL
That’s because Earth’s spin axis is tilted 23.5 degrees with respect to its orbit around the sun.

RACHEL
It means that anyone standing on either circles of latitude must see the sun pass straight overhead on the day of the solstice.

PAUL
The name for the northern circle of latitude is the Tropic of Cancer.

RACHEL
And the southern one is called the Tropic of Capricorn.

PAUL
The names Cancer and Capricorn were chosen because the sun appears in those constellations on the solstices.

RACHEL
These names were given to the tropics about 2,000 years ago

PAUL
And because the Earth’s spin axis slowly wobbles, the sun’s position today on the solstices has shifted by one constellation.

RACHEL
You can’t miss the line formed by Jupiter and Venus on the evening of the 20th.

PAUL
These two bright beacons are just calling for your attention.

RACHEL
Even better, the thin crescent moon appears to their left.

PAUL
Get binoculars out and look for look for Earthshine on the moon and Jupiter’s largest satellites.

RACHEL
That’s Idaho Skies for the third week of June. Next week is the start of Northern summer and we’ll tell you more.

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.