The Earth's shadow.

[jmeyer]

Hello,<br /><br />I have a question for anyone that can think of the answer.<br /><br />Who was the ancient astronomer that realized the Earth must be round, simply by using his imagination while witnessing an eclipse?<br /><br />

 

[Saiph]

Aristotle first wrote it down using observations of the earths shadow on the moon.<br /><br />Eratosthenes estimated the radius using the shadow of the sun at two location on the same day.<br /><br />Posidinious (sp?) used a star's maximum height in the sky from two locations to estimate the radius as well.<br /><br />Both Eratosthenese and Posidinious had similar answers, which are pretty close to the modern value. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

 

[jmeyer]

Thanks.<br />

 

[siarad]

<blockquote><font class="small">In reply to:</font><hr /><p>Eratosthenes<p><hr /></p></p></blockquote><br />I think it was he who also used a Greek 'pacer' to divine the distance between the two places. The resultant accuracy is a testament to the amazing consistency of the pacer too.

 

[Maddad]

Eratosthenes always amazed me by getting the size of the Earth so close to our modern value. He knew that on the first day of summer, when the Sun was highest in the sky, sunlight shone down the bottom of a well in the town of Sirene (sp?) in Egypt. This happened on no other day of the year. The town was located 23.5 degrees north latitude, which is why it happened. Because Eratosthenes knew that on this day the Sun cast a 7 degree angle in Alexandria to the north, he reasoned that the north-south component of the distance must be 360 / 7 or one 50th of a complete circle. He walked off the distance between the two towns, multiplied by 50, and came up with almost exactly 40,000 kilometers. Of course, his distance unit was stadia, not kilometers, but 50 times the distance between these two towns is about right.

 

[jmeyer]

But wasn't it Pythagoras who first taught it somewhere in the areas of 500 bc. This would put him centuries before Eratosthenes.<br /><br />I suppose it's near impossible to pin who said it first, but wouldn't most agree that Pythagoras gets the credit for this one?<br /><br />If I'm missing something, please let me know.<br /><br />Thanks.

 

[igorsboss]

<font color="yellow">the north-south component of the distance must be 360 / 7 or one 50th of a complete circle.</font><br /> <br />The arc component is 7/360. The distance multiplier is 360/7.

 

[Maddad]

I dunno jmeyer. Sounds like something worth looking up. All that I am reasonably sure of is that Eratosthenes said it. Pythagoras may have said it first, but that does not negate Eratosthenes having also said it.

 

[vogon13]

Speaking of shadows, has any one ever encountered the shadow of a jet contrail? Pretty weird experience. When you are close to one, it looks like a flat dark wall, and when you're in one it's noticeably darker. If you look up, the contrail will be blindingly bright around the sun due to forward scattering. They can move perpendicularly to the plane or at any angle. Some go quite fast. Encountering a near stationary one at a shallow angle in a car is especially strange. (Be care doing this, don't wreck your ride). Never heard any one talk about this. Just curious, thanx. <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[newtonian]

jmeyer - Centuries before Pythagors, the Bible recorded that the earth is round at Isaiah 40:22:<br /><br />(Isaiah 40:21-22) "There is One who is dwelling above the circle of the earth, the dwellers in which are as grasshoppers, the One who is stretching out the heavens just as a fine gauze, who spreads them out like a tent in which to dwell,"<br /><br />"Circle" is a translation of the Hebrew word "hhug" which means circle in 2 dimensions and sphere in 3 dimensions. <br /><br />Isaiah was written about 732 BCE (=BC).<br /><br />Many centuries earlier than that Moses wrote that the terminator on earth is a circle, which can only be true if the earth is a sphere:<br /><br />(Job 26:10) "He has described a circle upon the face of the waters, To where light ends in darkness."<br /><br />That was written about 1513 BCE, about 1,000 years before Pythagoras.<br /><br />Interestingly, earth is mostly water, and when light first reached earth's surface, earth was totally covered with water (see Genesis 1:2)

 

[igorsboss]

<font color="yellow">has any one ever encountered the shadow of a jet contrail?</font><br /><br />Yes, and it was amazing, because I looked UP into the shadow!<br /><br />The key to my experience was the thick haze in the air at the time. The sunlight was diminished by the haze enough so I could look up near the sun fairly comfortably.<br /><br />You've all had experience with seeing a sunbeam going through a dusty room. Well, what I saw was the opposite of that... A shadow going through a hazy sky.<br /><br />Usually, we see shadows on the ground. In this case, I saw the contrail shadow darkening the sky above me. I saw an incredibly flat 2-Dimensional plane of dark sky edge-on.<br /><br />As I figure it, the contrail's width must have subtended between 30 minutes and 1 degree of arc. The effect was similar to being in a total solar eclipse, but where the sun was eclipsed by a line, forming a plane of darkness in the hazy sky above me. Wow.