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--- courses:phy100:1 [2023/10/31 23:09] – [Terrestrial sphere] asad
+++ courses:phy100:1 [2023/10/31 23:22] (current) – [3.2 Celestial sphere] asad
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 From 3:33 in the video, I remove the earth from the picture and you see both hemispheres of the sky, albeit not at one glance. You are seeing the interior surface of the celestial sphere. The gridlines clearly show the two poles NCP and SCP and the hazy Milky Way appears as a circular band; normally we see only half of this band at a time, the other half being blocked by the horizons.
-You also see the shapes of the **constellations**. A constellation is a group of stars that form a recognizable pattern when viewed from the earth. These patterns were used for navigation by our ancestors and gave rise to numerous myths throughout human history. The patterns are of course imaginary. Some of the stars in a constellation might be very close to us, while some are extremely distant.  As the surface of the earth is divided into many countries and oceans, the surface of the imaginary celestial sphere is divided into 88 constellations.
+[[https://astro.unl.edu/naap/motion1/cec_both.html|{{:courses:phy100:ra-dec.webp?nolink|}}]]
-We identify people using their countries. Similarly, we identify celestial objects using the constellations in which they belong. For example, the Great Orion Nebula belongs to the Orion constellation. And here is a map of the Orion Constellation, the country of Orion.
-{{https://upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Orion_IAU.svg/825px-Orion_IAU.svg.png?nolink&600}}
-You see the borders and also its neighboring constellations; this is how we would show the map of Bangladesh as well with India and Myanmar as neighbors. Three stars (Alnitak, Alnilam, Mintaka) constitute the belt of the Orion. Betelgeuse and Bellatrix are the hands, Saiph and Rigel are the legs. Our ancestors devised a way to remember this region of the sky using the shape of Orion, a mythic hunter.
-Use https://stellarium-web.org for exploring various constellations. And one of the most curious thing you can observe is the Ursa Minor clock:
-{{youtube>nOlw30viFu8?large}}
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-The constellation Ursa Minor has 7 stars shaped like a bear as the ancients saw it. They look like a clock hand to me and if you rotate the sky for 24 hours in Stellarium, you see the hand move like the hour-hand of a 24-hour clock. This constellation was also called the Phoenician Bear. With the help of Ursa Minor, the mysterious Phoenicians navigated the Mediterranean sea and worked as the bridge from the Semitic aleph-ba-ta to Greek alpha-beta and finally to the world-wide alphabet.
 ===== - A tour in Time =====