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--- courses:phy100:2 [2023/10/06 03:48] – asad
+++ courses:phy100:2 [2023/10/31 23:23] (current) – [5. Constellations] asad
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 ====== 2. Sky as a cultural heritage ======
-===== - Constellations =====
 ===== - Seasons and the sun =====
 There are four seasons in a year: Spring, Summer, Autumn and Winter. Temperature is very high in summer, low in winter, but spring and autumn seem to have an optimum comfortable temperature. Why?
-The seasons change because of the tilt of earth's axis and its revolution around the sun. The following video made using [[https://astro.unl.edu/naap/motion1/animations/seasons_ecliptic.html|this simulator tool]] helps explain.
+{{https://c.tadst.com/gfx/1200x675/equinoxes-and-solstice.png?nolink&600}}
+The seasons change because of the tilt of earth's axis and its revolution around the sun. As the earth goes around the sun, the times when the axis is most tilted toward or away from the sun are called **solstices** and the times when the axis is tilted neither toward nor away from the sun are called the **equinoxes**. It is either spring or autumn around the times of the equinoxes, and summer or winter during the solstices, as you see above.
+The following video made using [[https://astro.unl.edu/naap/motion1/animations/seasons_ecliptic.html|this simulator tool]] helps explain in more detail.
 {{youtube>U7sK-_pMUhQ?large}}
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 \\
 Let us first see the video of a total solar eclipse. The moon encroaches on the sun and we see the a circular disk sliding on top of the sun. As the sun
+===== - Constellations =====
+You also see the shapes of the **[[uv: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.
+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}}
+\\
+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.