Model - Sun, Earth and Moon

  Let us understand various events associated with Sun, Earth and moon with the help of a model.

Model is built with small balls, beads and lots of gears. Let us assume that we are way up in the sky and observing these objects from a pole star. Earth revolves around the Sun, not in clockwise direction but in anti clockwise direction.

During this journey around the Sun, it also rotates around its own axis. This axis is not straight but slightly tilted at an angle of 23.5 degrees like this. This tilt is the reason we have seasons on Earth.

Just like it's revolutions around the Sun, Earth also rotates in anti-clockwise direction around it-self. One complete rotation around itself takes 24 hours while one complete revolution around Sun is finished in 365 days or 1 year. Let's take help of this dial so that position of Earth can be referenced as it revolves.

There are numerous stars around our solar system. These were grouped by our ancestors for easier identification. Also know as constellations. There are many but 12 are most important. These constellations mark out the path that Sun appears to take throughout the year.

Mostly known to us as Zodiac signs. Circle is divided into 12 equal parts. One part per zodiac sign.

This outer ring has months marked on it as per Gregorian calendar. Useful while discussing Seasons.

Zodiac signs are arranged like this… Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces.

And months as January, February, March, April, May….

Let's move Earth by hand in steps and study orientation of axis as it revolves around the Sun. Front view is better for this instead of top view

This imaginary line represents Equator. It divides Earth in two equal parts. Northern hemisphere and Southern hemisphere.

Let us start with 21st June.

21st June is the longest day in Northern hemisphere. North pole is at its nearest point towards Sun on this day. Hence this part gets more sunlight. Longest day and shortest night.

Exactly opposite situation is there in the southern hemisphere. Longest night and shortest day .

It's also known as summer solstice.

22nd September

On this day Sun is exactly above the Equator, which makes day and night of equal length. Marking the start of Autumn. During the autumnal equinox, the sun shines directly on the equator, and the northern and southern hemispheres get the same amount of rays.

22nd December

On this day North Pole is farther from the Sun. Northern hemisphere receives less Sunlight.

Days are shorter and nights longer. It's also known as winter solstice. Exactly opposite situation is there in Southern hemisphere.

21 March

Earth’s two hemispheres are receiving the sun’s rays equally. Night and day are often said to be equal in length. Also known as spring equinox or vernal equinox.

Sun rises southward every day starting 21st June till 21st December in Northern hemisphere. And starts rising northward again from 22nd December. These two periods are Popularly known as Uttarayan and Daxhinayan in India.

Moon

This small plastic bead is our Moon.

While the Moon revolves around the Earth, Moon and the Earth revolves around the Sun.

It takes approximately 27.3 days for Moon to complete one revolution around the Earth.  But Earth is also moving constantly. Moon takes more time to reach the same spot. 29.5 days.

The Moon itself does not generate light; it is lit up by the Sun. As the Moon orbits the Earth, the portion of illuminated Moon that we see changes – giving rise to the phases of the Moon.

The Moon does not produce its own light. There is only one source of light in our solar system, and that is the Su. “moonlight” is actually just sunlight reflecting off of the Moon’s surface. The Sun’s light comes from one direction, and it always illuminates, or lights up, one half of the Moon – the side of the Moon that is facing the Sun. The other side of the Moon is dark.

On Earth, our view of the illuminated part of the Moon changes each night, depending on where the Moon is in its orbit, or path, around Earth. When we have a full view of the completely illuminated side of the Moon, that phase is known as a full moon.

In this video, I short recently on a full moon day, moon is seen rising in the east while sun sets in the west. But following the night of each full moon, as the Moon orbits around Earth, we start to see less of the Moon lit by the Sun. Eventually, the Moon reaches a point in its orbit when we don’t see any of the Moon illuminated. At that point, the far side of the Moon is facing the Sun. This phase is called a new moon. During the new moon, the side facing Earth is dark.

We have a “new Moon” when our Moon's orbit around Earth moves it between Earth and the Sun. From Earth, the Moon's surface looks dark because the illuminated side is facing away from Earth. As our Moon continues its orbit counterclockwise around Earth (viewed from above the north pole), more and more of the illuminated part of the Moon becomes visible to us, until it reaches the “full Moon” stage. A full Moon occurs when the Moon has moved in its orbit so that Earth is “between” the Moon and the Sun.

When the Sun and Moon are aligned on the same side of the Earth, the Moon is "new", and the side of the Moon facing Earth is not illuminated by the Sun.

A waxing moon is a moon that gets more sunlight on it as the days go by. It is after we experience a new moon up to a full moon. A waning moon is after the full moon and will remain waning until we again experience a new moon.

Eclipse is another event familiar to us.

A lunar eclipse occurs when the Moon moves into the Earth's shadow.

A solar eclipse occurs when a portion of the Earth is engulfed in a shadow cast by the Moon which fully or partially blocks sunlight. This occurs when the Sun, Moon and Earth are aligned. Such alignment coincides with a new moon

If the Moon were in a perfectly circular orbit, a little closer to the Earth, and in the same orbital plane, there would be total solar eclipses every new moon. However, since the Moon's orbit is tilted at more than 5 degrees to the Earth's orbit around the Sun, its shadow usually misses Earth. A solar eclipse can occur only when the Moon is close enough to the ecliptic plane during a new moon.

Due to tile in the orbital planes of Moon and Earth, eclipses do not occur every 15 days.

Lets use this small dial to help is identify various phases of moon as it revolves around the Earth.

Same side of the moon

As speed of rotation of moon around its own axis and time it takes for one revolution around Earth is same, we always see same side of the moon. Let us mark a red line here. It is always facing the earth.

Makar Sankranti

We celebrate Makar Sankrant every year around 14th or 15th Jan. On this day, Sun enters from Dhanu rashi to Makar Rashi. But what does enter a zodiac means. ?

Sun is stationary just like many constellations around us. When observed from Earth, Sun appears to be in particular constellation in the background. This keeps on changing as Earth revolves around the sun. When these imaginary boundaries are crossed, we refer to it as SUN entering a particular constellation/ or zodiac sign or Rashi.

Most of the hindu festivals are celebrated as per Lunar calendar but Makar Sankrati is based on the transition of SUN. This is why it normally comes around 14th or 15th Jan of Gregorian calendar.

In the next video, we will discuss about Hindu calendar which considers lunar as well as solar position while deciding months.

Thank you.