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--------------------------- - - 1946 - Full Moon Daylight Savings
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- How big is the Moon? Well, it is about as wide as your thumb held at arm’s length. It is about ½ degree. A circle is 360 degrees of arc. There are 60 arc-minutes in a degree, and , 60 arc-seconds in an arc-minute. Horizon to horizon is 180 degrees. So you could set 360 Moons touching edge to edge in a giant arc across the night sky.
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- At 20 feet the little letters on an eye chart would appear 1/6th the width of the Moon. Assuming you had 20/20 eyesight. Those little letters displace an angle of 5 arc-seconds. To read the letters you need an arc-minute of 60 arc-seconds of seeing resolution. So, 20/20 vision is defined as having one arc-minute of seeing resolution.
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- A single arc-minute is 1/30th the width of the Moon. The Moon is 2,160 miles across. Your eye can resolve 72 miles at that distance. You could not see a lunar city on the Moon’s surface if there was one.
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- Of course the 20/20 vision test is done in bright light. You can resolve one arc-minute. But, in the night sky, observing the Moon and the stars you can likely only resolve 3.5 arc- minutes.
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- An arc- second is 1-60th of an arc-minute. It is the size of a dime seen 2.5 miles away. Now, this is where a telescope comes in. If its magnification is 60X, then an arc-second becomes an arc-minute.
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-- For example: Saturn’s rings are 0.5 arc-seconds across. 120X telescope power would make it one arc-minute across. Venus is one arc-minute across, that is 60 arc-seconds. Jupiter is 50 arc-seconds wide. Mars is 24 arc-seconds in the year 2018.
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- Uranus is 4, Neptune is 2, and Pluto is 1 arc- second. To see Pluto you need a 600 power telescope. But, air turbulence at that magnification makes it impossible to see Pluto.
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- Betelgeuse star is 1/20th of an arc-second. This happens to be the current limit of the Hubble Space Telescope. Sirius and Alpha Centaurus are closer stars and 1/200th of an arc-second. No telescope can resolve their diameter. What we see is their star-light that shines brightly in the night sky. Count all the stars one night. If you get to 3,000 you have reached all that you can see at one time in the night sky with you naked eyes.
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- But, each night new stars and planets come into view as the night goes on , and the month’s transpire. We see a new picture as the Earth circles around the Sun. I know this is a lot of arcs, but, I just thought you should know this. It only takes about 3 minutes to read., unless you are working in arc-minutes then it takes about 60 scans and you have to turn the page.
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----- 707-536-3272 ---------------- Saturday, March 11, 2017 -----
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