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--------------------- 2662 - ASTRONOMICAL - distances and directions?
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- Our four seasons are caused by the Earth being tilted 23.4 degrees from its orbital plane as it circles the Sun. The north Celestial Pole is 23.4 degrees to the north of the Ecliptic Pole.
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- This has the same effect as tilting our equator. The Celestial Equator , the one that goes around our globe, is at a 23.4 degree angle to the ecliptic, the plane of our rotation around the Sun.
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- The Spring Equinox marks the spot on the Equator where the length of the day and the length of the night are the same, 12 hours. This year that occurs on Thursday, March 19 2020.
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- That point is put at zero angular degrees and the equator around the globe is divided into 360 degrees. Each degree is divided into 60 minutes and each minute is divided into 60 seconds. These are angular degrees, minutes and seconds and not to be confused with time.
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- The east-west coordinates are the celestial equivalent to longitude on the Earth.
But, in astronomy it is called the Right Ascension, not longitude The Right Ascension is measured eastward in hours, minutes, and seconds of time, starting at zero at the Spring Equinox.
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- Since the Earth rotates 360 degrees in 24 hours, this equates to a one hour “Right Ascension” being 15 degrees of arc. One minute of time equates to 15 minutes of arc. And, one second of time equates to 15 seconds of arc.
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- The Spring Equinox, or Vernal Equinox, marks the point at which the Sun, therefore the ecliptic plane, crosses the celestial equator on its way north. The north-south coordinates are called “Declinations”, and that is equivalent to latitudes on the Earth’s surface. It is measured from 0 to 90 degrees north and 0 to 90 degrees south to get to each of the celestial poles.
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- With these definitions we can find celestial objects that are cataloged at a particular time to be in a particular place in the sky.
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- For Example: Norton’s 18th addition published for December, 2000 said that Pleiades, open cluster in Taurus, Messier 45, would be at Right Ascension 03 hours, 47 minutes and Declination + 45 degrees, 07 minutes.
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- Of course these directions need to be published each year to be perfectly accurate. The Earth’s orbit is not perfectly repeatable and the Earth’s tilt precesses so that north pole does not always point towards the North Star.
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- With these directions you can use your hand at arms length to estimate your degrees pointing around the sky. Your fist at arms length is 10 degrees. Your index finger is 2 degrees of arc. The Sun and the Moon are ½ degree of arc.
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- There are many other guide posts in the night sky once you become familiar with the constellations. It just takes some practice.
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- Astronomer’s measure distance in lightyears, the distance light travels in one year at 186,282 miles per second. The velocity of light is in the metric system is 299,792,458 meters / second. The distance is 9.46 * 10^12 kilometers, or 5,880,000,000,000 miles in one lightyear.
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- Astronomers measure the brightness of stars in Apparent Magnitude of Luminosity. Apparent because you do not know the Absolute brightness unless you know how far away the star is. The further away the dimmer the star appears.
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- The Magnitude metrics all started with the Greeks when they assigned the brightest star a magnitude of 1 and those stars just barely visible to the naked eye a Magnitude of 6. Later, once technology took over the measurements, each step in Magnitude was defined as a factor of 2.512 times.
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- Therefore, 1 to 6 Magnitudes, being five steps, equates to (2.512)^5 = 100. The brightest star and the dimmest star we can see has a factor of 100 difference in brightness.
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- The bright star Vega was given a 1 and all other stars were compared to Vega. If a star was brighter than Vega it was given a negative number. Sirius is -1.46 Magnitude. Venus is -4 Magnitude. The Sun is -27 Magnitude.
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- Given the distance to a star with an Apparent Magnitude measured you can easily calculate the Absolute Magnitude or the Luminosity of that star. The light spreads out as the surface of a sphere. The area of the surface of a spheres is 4*pi*r^2. The distance being the radius.
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- So, the star’s Luminosity in watts = (4*pi*r^2) * (Apparent Magnitude) in watts per square meter. Or, if you know a stars actual luminosity through some other means you can calculate its distance using the same formula.
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- It just takes practice with a star map and pretty soon you have a whole new world in your backyard.
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- March 10, 2020 902 2662
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--------------------- Tuesday, March 10, 2020 --------------------
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