Ariviyal Arangam - Science Forum
Lecture series, Part 1, 2 & 3 for students and adults by D. Navaneetham PhD
Periyar Nagar public library
(Located near Karthigeyan and Kandasamy Salai junction, Chennai 82)
Organized by Mr. Venkatesan, Librarian and Coovam Lizards
Astronomy - Part 1 - Understanding the night sky - April 19, 2015, Sunday 11 am
Proceedings at the session compiled by Anandhi Naryanaswamy MSc., MS.
Order out of chaos
Since time immemorial humans have always been trying to make sense of the world around them. In an effort to bring some organization into the seemingly disorderly starry night sky overhead, our ancestors looked for patterns in the distribution of stars in the sky. The resultant join-the-dot kind of order was constellations.
Constellations, zodiac and planets in the night sky
Imagine the night sky as a black sphere around the Earth, with stars and planets represented as shiny white lights on this celestial sphere (in reality, these celestial objects are distributed in 3-dimensional space). There are 88 constellations (pattern of stars) in the night sky observable from different locations on the Earth. Twelve of these constellations along the ecliptic band (path of apparent movement of planets and Sun in the sky) constitute the Zodiac or Raasi Mandalam. Planets are visible as slightly brighter objects in the night sky.
The science part of Panchangam (Indian almanac)
The zodiac part of the celestial sphere encompasses 360 degrees. On dividing this among 12 rasis, each rasi is equivalent to 30 degrees. Each rasi contains 2 to 3 representative stars or natchathirams, with each star taking up about 13 degrees each. Each star is further subdivided into 4 padhams – each padham measuring approximately 3.3 degree. Thus there are 108 padhams in 360 degree raasi mandalam. The position of the moon in the zodiac or ecliptic at the time of birth of a person determines his or her birth star and birth rasi.
In the raasi mandalam chart of panchangam, each rasi is represented as a house. Every year in the month of Chithirai, the Sun enters the zodiac constellation of Mesham, the Aries. For the rest of the year, the Sun continues to traverse the Zodiac, a rasi a month, and finishes the passage in Meenam, the Pisces. It restarts the cycle for the subsequent year again from Mesham.
While the position of stars in their constellations in the sky seems fixed, planets (which are much closer to the Earth, relative to the stars), wander in a specified elliptical path in the sky against the backdrop of stars. This linear movement of planets from one constellation to another along the Zodiac is called Grahappeyarchi in Tamil. For instance, Gurupeyarchi of Jupiter happens every year as Jupiter takes one year to traverse a rasi and 12 years to complete its revolution around the Sun, while Sanipeyarchi of Saturn takes two-and-half years and its revolution 30 years.
Practical observation of night sky
If you observe the night sky from a open area at this time of the year (April-May), around 7 pm, at Chennai (13° North latitude), you can easily spot the non-zodiac constellations of Orion, Canis Major, Canis Minor, Ursa Major and the zodiac constellations of Taurus, Gemini, Leo, Virgo.
Apart from constellations, there are other discernable patterns in the sky called asterisms. An asterism might involve multiple constellations or just be part of one constellation. An example of the former is Big Dipper – a group of 7 stars – also known as Saptharishi Mandalam (Figure 3), while an example of the latter is the Hexagon comprising of stars from the constellations of Canis Major, Canis Minor, Gemini, Auriga, Taurus and Orion.
For graphic descriptions visit: https://sites.google.com/site/coovamlizards/explore/astronomy
Astronomy – Part 2 - Celestial Motions and Consequences – May 3, 2015, Sunday 11 am
Proceedings at the session compiled by Anandhi Naryanaswamy MSc., MS.
Movement of planets
Rotation and duration of day All celestial objects including our planets of the Solar System rotate on their axes. Earth rotates on its axis in a 24-hour time period causing day and night. Each object has its own rotation period. Venus for example takes about 243 days to make a complete spin on its axis, while sun takes 25 days.
Revolution and seasons All planets in our Solar System revolve around the Sun at varying speeds in elliptical orbits. This revolution speed is what helps the planets fight the enormous gravity of the sun and keep them from falling into the Sun’s gravitational field. The speed and duration of revolution of a planet around the sun is determined by the distance of the planet from the Sun as also the planet’s mass.
Additionally Earth’s rotational axis is tilted at 23.5° from its vertical. During the 365 day revolutionary time period of the Earth around the Sun, this tilt causes an apparent migration of the Sun between latitudes 23.5° N (Tropic of Cancer) and 23.5° S (Tropic of Capricorn). This inclination on its axis causes the gradual change in angle of incidence of sun’s rays, causing seasons like summer, winter, spring and autumn on earth. The apparent northward journey of the Sun to the Tropic of Cancer is traditionally named Uttarayanam (when the Earth’s northern hemisphere is tipped towards the Sun and regions here experience summer) and the southward journey of the Sun to the Tropic of Capricorn is traditionally named Dakshinayanam (when the Earth’s southern hemisphere is tipped towards the Sun and regions in southern hemisphere experience summer).
If Earth’s did not have the 23.5° tilt on its axis and consequently the Sun was to remain overhead at the equator throughout the year, any part on the Earth would have the same climatic conditions throughout the year. (Figure 1).
Phases of the Moon
Moon being a non-luminous object appears bright in the night sky only because it reflects light from the Sun. At any time, one half of the moon is always lit. But, when seen from Earth, depending on the position of the Moon with respect to Earth and Sun, we either see a wholly lit Moon (full moon or Pournami), an unlit moon (new moon or Amavasai) or partially lit faces of the moon (waxing moon or Valarpirai, and waning moon or Theipirai). (Figure 2).
Eclipses of the Sun and the Moon
Whenever the Sun, Earth and Moon line up in the same plane, we end up with an eclipse – solar or lunar – depending on whether the Earth or Moon gets in the middle. When the Earth comes between the Moon and the Sun, Earth’s shadow on the Moon causes a Lunar Eclipse (eclipse means to hide). When the Moon comes between Sun and Earth, Moon’s shadow on earth causes a Solar Eclipse by blocking out the light from the Sun.
So what prevents a lunar eclipse from occurring on every full moon or a solar eclipse from occurring on every new moon?
The orbit of the moon (around the earth) is inclined to the ecliptic (plane of the Earth’s orbit) at about 5°. The points where the Moon’s orbit crosses the ecliptic are called lunar nodes. There are two such nodes – the ascending node and descending node. Traditionally ascending node is called Rahu and descending node is Kethu. Only when the Moon passes through a node during new moon does a Solar Eclipse occur. Similarly when Moon passes through the node during a full moon, a Lunar Eclipse occurs. (Figure 3 and 4).
Transit of planets between the houses of the zodiac
Each rasi mandalam or zodiac sign spans approximately 30° on the celestial sphere. Together all 12 rasi mandalams occupy a path encompassing 360° on the celestial sphere (12X30°=360°). When a celestial object, say Jupiter (observed against a background of the zodiac) passes from one rasi to the next, it is called planetary transition (Grahapeyarchi).
Each planet has its own transition duration depending on the duration of its revolution around the sun. In the case of Jupiter, 12 years are required to complete one revolution around the sun and therefore it dwells in a rasi for a whole year. And for Saturn the transition occurs approximately every 2.5 years, since its one revolution around sun requires 30 years (12 rasi x 2.5 years=30).
Relevance of Agni Natchatiram
A stretch of hottest period (22-23 days) during the summer (Chithirai-Vaikasi or May) is referred to as Agni Natchathiram in southern India. When the sun is at halfway through its journey of Uthrayanam, around March 21, it shines directly overhead on the equator (0° latitude). As the sun continues to make the northward journey to Tropic of Cancer, it begins to pass overhead on southern India around the month of May. As a result the land mass and the air above it warm up. During this time sun traverses the star Kiruthigai (about 13° within the Zodiac sign Aries or the rasi house of Mesham) and parts of the adjoining stars Bharani and Rohini in the same Zodiac sign Aries. Kiruthigai is sometimes also called Agni Natchathiram.
During the Sun’s southward journey during Dhakshinayanam that starts around June 21, it again passes overhead the Tamil land during August. But by then the southwest monsoon sets in, cooling down the land and air, so the heat isn’t as intense as during the month of May. Therefore Agni Natchathiram is not only geography specific (as we experience it only in extreme southern India), but is also influenced by local weather.
For graphic descriptions visit: https://sites.google.com/site/coovamlizards/explore/astronomy
Astronomy – Part 3 - Structure of the Universe – May 10, 2015, Sunday 5 pm
Proceedings at the session compiled by Anandhi Naryanaswamy MSc., MS.
Proceedings at the session
Moving away from Earth into space, this is what we’d encounter – the planets in Solar System, the planetesimals like asteroids, comets and meteoroids, Sun, stars, nebulae and galaxies. While within the Solar system, distance is measurable in astronomical units or AU (1AU is distance from Earth to Sun, ie., 150 million kilometers), distance between stars and galaxies are measured in light years. One light year (1ly = 9 trillion km) is the distance travelled by light in one year at the speed of 300,000 km per second.
Planets of the Solar System
The four planets closest to the Sun, also called the inner planets – Mercury, Venus, Earth and Mars – are terrestrial planets, made up of dense, rocky materials. The outer planets also called Jovian planets – Jupiter, Saturn, Uranus, Neptune (Pluto has been classified as a dwarf planet since 2006) - are made mostly of gases like hydrogen and helium. While among the inner planets, Earth has one moon and Mars has two, the outer planets due to their bigger size, heavier mass and greater distance from the Sun have captured a great number of natural satellites to orbit around them. Jupiter has 63 so-far identified moons, Saturn about a hundred moons, Uranus around 23 moons and Neptune has 14 known moons.
Asteroids, Comets and Meteorites
A large population of debris leftover from the formation of Solar System form asteroids, comets and meteoroids. While asteroids are small rock bodies orbiting the Sun in the region between Mars and Jupiter, the comets (Vaal Natchathiram) are a mixture of ice, dust and rock that lie beyond the orbit of Pluto in a region called Oort cloud. Fragments from asteroids or comets are called meteoroids. These on entering Earth’s atmosphere form a streak of light called meteor or shooting star (Eri Natchathiram). Most of these meteors burn up in atmosphere, but the ones that manage to land on Earth as charred rocks are called meteorites.
Stars and Nebulae
Besides the moon and few planets, our night sky is cluttered with millions of stars. Stars, unlike planets, are self-luminous bodies. Their luminosity results from the energy of nuclear fusion reaction taking place in their interior. Stars are formed within nebulae which are stellar nurseries. Depending on a star’s mass and its stage in life, it can be classified as Supergiant, Giant, Main Sequence and Dwarf. A star’s temperature determines its color – blue being the hottest and red being relatively cooler. Sun is a main sequence yellow-orange star, about 4.5 billion years old and about half-way through its life.
Galaxies and Clusters
Billions of stars along with dust and gas which are held together as one single system due to gravity are called galaxies. Our own Earth in its Solar system belongs to the galaxy called Milky Way. On dark nights from remote places away from city lights, part of this Milky Way (Aagaya Gangai or Paalvazhi Mandalam) can be seen as a hazy white band across the sky in the east-west direction.
The center of the Milky Way is known to have a Black Hole called Sagittarius A*. Black Holes are regions in space, where the gravity is so immense that even light cannot escape. Black holes therefore are not visible but can only be inferred by their gravitational influence on their neighborhood.
Galaxies are classified according to their shape as Elliptical, Spiral or Irregular galaxies. While Milky Way and its famous larger neighbor Andromeda are spiral galaxies, our closest neighboring galaxies, the Small and Large Magellanic Clouds are examples of irregular galaxies.
Groups of galaxies exist together in clusters. Our own Milky Way along with about 29 other nearby galaxies is part of a cluster called the Local Group. Multiple such clusters themselves form a Supercluster. A famous example would be Virgo Supercluster of which our Local Cluster is a part.
Universe and its origin
Universe includes all of space, time and matter within. Its size is not known. As seen from our own Local Group, all galaxies beyond are moving away from us. Scientists deduce the Universe is expanding. According the widely accepted Big Bang Theory, about 15 billion years ago, all matter was held in a singularity (when all physical laws known to man break down – there is neither time nor space nor matter). At the moment of the Big Bang explosion, Universe was created and has been expanding since. 10 seconds after the big bang, sub-atomic particles had been formed. Several hundred years later, atoms coalesced from these particles. About 1-3 million years later, these atoms clump into clouds of gas that will become stars. Around 100 million years later, the first stars are formed. Some 9 billion years after the big bang, Milky Way galaxy comes together – this includes the Sun and its planets.
For graphic descriptions visit: https://sites.google.com/site/coovamlizards/explore/astronomy
