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--- courses:phy100:1 [2023/10/06 02:06] – asad
+++ courses:phy100:1 [2023/10/31 23:22] (current) – [3.2 Celestial sphere] asad
@@ Line 2: / Line 2: @@
 Can you see the connection between English 'star' and Bangla 'স্থির'? And why do English 'love' and Bangla 'লোভ' sound similar, and how do they relate to 'লুব্ধক', the brightest star in the sky called 'Sirius' in English?
-===== - Space-time and matter-energy =====
+===== - Space-time and energy-matter =====
-In the year 2005, Einstein discovered two surprising facts about the universe we live in when he was only 26 years old. The first was simply that space and time are the same thing:
+In the year 2005, [[wp>Einstein]] discovered two surprising facts about the universe we live in when he was only 26 years old. The first was simply that [[wp>space]] ($S$) and [[wp>time]] ($T$) are the same thing:
-$$ \text{Space} \equiv \text{Time}. $$
+$$ \text{Space} = \text{Time}. $$
-The second is that matter and energy are two forms of the same thing. This was formalized in his famous equation $E=mc^2$ which we will write in using natural language as
+The second is that [[wp>matter]] ($M$) and [[wp>energy]] ($E$) are two forms of the same thing. This was formalized in his famous equation $E=Mc^2$ which we will write in English as
 $$ \text{Energy} \equiv \text{Matter}. $$
+We measure space using the unit **meter** ('m' in short), time using **second** ('s'), energy using **joule** ('J') or **watt** ('W') and matter using **kilogram** ('kg').
+{{:courses:phy100:spacetime.png?nolink|}}
+The universe we observe from Earth can be thought of as a spherical ball (shown above) of spacetime filled with energy-matter. Here, space is confined to the surface and time to the radius, but they are connected. We are at the center of the sphere, and we can see a sphere at every radius which is basically the distance of the spherical surface from us. In 1915, Einstein found another surprising fact through his mathematical analysis:
+$$ \text{SpaceTime} \sim \text{EnergyMatter}. $$
+Space-Time (ST) and Energy-Matter (EM) are intimately related. EM tells ST how to bend, and ST tells EM how to move.
 ===== - A tour in Space =====
@@ Line 70: / Line 80: @@
 ===== - Maps of Space =====
+==== - Terrestrial sphere ====
 The curved surface of the spherical earth (the **terrestrial sphere**) is the space we will start to map first, as an orientation for this course. Since Aristotle described the round shadow of earth on the surface of the moon, no astronomer ever doubted that the earth is round. We have mapped the 2-dimensional surface of this sphere using a **geographic coordinate system** that has two references and two angles of measure: latitude and longitude. See the video first.
@@ Line 84: / Line 95: @@
 Can you see the similarity with the example of the marker? The two references here are like the two walls in the marker example. And the angles here are equivalent to those tile-blocks. Here we do not need the third dimension because we are not interested in measuring heights from the surface.
+[[https://astro.unl.edu/naap/motion1/tc_both.html|{{:courses:phy100:lat-long.webp?nolink|}}]]
+==== - Celestial sphere ====
 {{:courses:phy100:twospheres.png?nolink|}}
@@ Line 100: / Line 114: @@
 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}}
-\\
-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 =====