Differences

This shows you the differences between two versions of the page.

--- courses:phy100:2 [2023/06/10 06:42] – [3.1 Lunar eclipse] asad
+++ courses:phy100:2 [2023/10/31 23:23] (current) – [5. Constellations] asad
@@ Line 1: / Line 1: @@
-====== 2. Sky as Cultural Heritage ======
+====== 2. Sky as a cultural heritage ======
 ===== - 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}}
@@ Line 50: / Line 54: @@
 Second question: during a full moon, why the moon is not completely blocked from sunlight by earth? if the earth is between the sun and the moon, shouldn't the moon be completely in the shadow of earth if they are really aligned? Yes, but the fact is the plane of moon's orbit is not exactly aligned with the orbital plane of earth, they have an angle of 5 degrees as shown below.
-{{https://upload.wikimedia.org/wikipedia/commons/thumb/4/46/Lunar_Orbit_and_Orientation_with_respect_to_the_Ecliptic.svg/1024px-Lunar_Orbit_and_Orientation_with_respect_to_the_Ecliptic.svg.png?nolink}}
+[{{https://www.virtualtelescope.eu/wordpress/wp-content/uploads/2017/07/MoonOrbitEclipses.jpg?nolink&800|Credit: https://www.virtualtelescope.eu.}}]
 On rare occasions, the sun, the earth and the moon really align creating a lunar eclipse as we will see in the next section.
@@ Line 64: / Line 68: @@
 **Earthshine**: during the crescent phases of the moon, the sunlight reflected from earth illuminates the dark part of the moon. This is called earthshine, first described by Galileo in 1610. If you are in a dark location, you can sometimes see the dimly illuminated full moon along with a bright crescent. The crescent is caused by direct sunlight and the the remaining part by the sunlight reflected from earth.
-===== - Eclipses =====
+**Lunar cycle:** If you have not thought about the influence of the moon on us, just think about the menstrual cycle in women((The connection between lunar cycle and menstrual cycle is not direct and it is also a matter of debate among researchers, and the connection might have weakened due to our modern lifestyle. Maybe ancient humans had better connection with the moon than us.)) and the sleep cycle common to both men and women. People sleep less during full moon, more during new moon. The moon takes around 27.5 days to go around the earth once but the phases complete one cycle in around 29.5 days. There is a 2-day difference because by the time the moon completes one orbit, the earth moves forward in its orbit around the sun. So the moon takes 29.5 days to come back to its initial position with respect to the sun.
-==== - Lunar eclipse ====
+===== - Lunar eclipse =====
 On the rare occasions when the sun, the earth and the moon completely align during a full moon, we get lunar eclipse, that is we see the shadow of the earth on the surface of the full moon, and the moon gets darker than usual.
@@ Line 82: / Line 87: @@
 The reason is the same as the reason behind seeing a **blue sky**. Sunlight is white, it has all the 7 colors of rainbow, but blue light is disturbed by the atmosphere of earth more than red light. When the sun is up, the red light reaches earth's surface without much interruption but the blue light is scattered throughout the sky by the molecules of air; so the sky is blue.
-During a total lunar eclipse, sunlight is not completely blocked, some light from the sun refract (bend) due to earth's atmosphere and then shines on the moon. As the atmosphere scatters the blue light away, only the redder lights can go to the moon uninterrupted. This is why the moon looks reddish during an eclipse and called the '**blood moon**' giving an eerie feeling.
+During a total lunar eclipse, sunlight is not completely blocked, some light from the sun refract (bend) due to earth's atmosphere and then shines on the moon. As the atmosphere scatters the blue light away, only the redder lights can go to the moon uninterrupted. This is why the moon looks reddish during an eclipse and called the '**blood moon**' giving an eerie feeling. See the timelapse video of a lunar eclipse of 2018.
-==== - Solar eclipse ====
+{{youtube>buXTecdfqxo?large}}
+\\
+Question based on the video: why does the moon turns red only after it is completely eclipsed by earth?
+The blood moon of lunar eclipse and the black sun of the solar eclipse had a huge influence on the psychology of human cultures. Here is the Bible: "And I beheld when he had opened the sixth seal, and, lo, there was a great earthquake; and the sun became black as sackcloth of hair, and the moon became as blood; And the stars of heaven fell unto the earth, even as a fig tree casteth her untimely figs, when she is shaken of a mighty wind." --- Book of Revelation, New Testament.
+===== - Solar eclipse =====
+Sun goes completely black during a total solar eclipse, when the moon is between the earth and the sun during a new moon with perfect alignment of the three celestial objects.
 {{https://upload.wikimedia.org/wikipedia/commons/thumb/0/00/Geometry_of_a_Total_Solar_Eclipse.svg/768px-Geometry_of_a_Total_Solar_Eclipse.svg.png?nolink&500}}
-This website has maps of all total and annular solar eclipses within 5000 years from 2000 BCE to 3000 CE: https://eclipse.gsfc.nasa.gov/SEpubs/5MCSE.html.
+Again you can see the umbra and penumbra created by the sun and the moon on earth. Earth's shadow falls on the moon during a lunar eclipse, but there is no shadow in a solar eclipse, the very source of light is completely blocked by a piece of grey rock, the moon.
-{{https://eclipse.gsfc.nasa.gov/SEatlas/SEatlas3/SE2001-25T-2.GIF}}
+There is another big difference between the lunar and solar eclipses. A total lunar eclipse can be seen from many places on earth for almost an hour, but a total solar eclipse can only be seen within a 300-km wide patch on the surface of earth and only for a few minutes even from there; you have to be within the patch to see the total eclipse. This is basically the umbral 'shadow' of the moon on the earth.
-{{https://eclipse.gsfc.nasa.gov/SEatlas/SEatlas3/SE2001-25A-2.GIF}}
+You can find the time, duration and locations of the solar eclipses for the next 10 years here: https://www.timeanddate.com/eclipse/list.html. This website has maps of all total and 'annular' solar eclipses within 5000 years from 2000 BCE to 3000 CE: https://eclipse.gsfc.nasa.gov/SEpubs/5MCSE.html. What is an 'annular eclipse' will be clear shortly.
-===== - Comets and novae =====
+{{youtube>HM5w7Aq8buw?large}}
+\\
+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
-===== - Timekeeping =====
+===== - 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.