Astronomy Basics - Interesting Facts About Earth, Sun, and Moon
Astronomy is the branch of science that deals with celestial objects, space, and the physical universe. The three major celestial objects that have a discernible impact on our lives are the Sun, Moon and our own Earth. Understanding the basic functioning of Earth, Sun, and the Moon is the appetizer before the main meal.
Earth and Moon are part of a gravitational system where Moon goes around Earth in 27 days. The planets and minor planets are part of a larger gravitational system bound to Sun, known as the solar system. Earth takes over 365 days to complete its orbit around the Sun.
Earth is a spinning round ball. There are many pictures of Earth taken from space showing our planet as a round ball. Have you ever wondered why the planets, Moon, and the Sun are round?
In large objects such as planets or stars, gravity bounds everything together. If gravity is strong enough, the mass gets uniformly distributed around its center, resulting in a round shape.
Our planet is spinning on an imaginary axis that runs between Polaris, the North Star, and the South Pole. So the stars near the poles appear stationary when viewed from Earth.
Day and Night
At any moment, one half of Earth’s surface receives sunlight while the balance half does not receive any sunlight. It is daytime for the regions receiving sunlight and night for the balance regions.
We know that day and night keep alternating in a region. This can happen if the ball of Earth is spinning. When it completes half spin, the regions having a day earlier would have night and vice versa. Thus, the Earth’s spin causes the never-ending cycle of day and night. It takes 24 hours for the Earth to complete one spin.
USA and India are on either sides of the globe. So when it is midday in New Delhi, it is midnight in Washington and vice versa. When one makes a long distance call between the two regions, they can appreciate that Earth is a spinning round ball because of the difference in local time.
Can We Feel the Spin of the Earth?
We cannot feel the spin just as we do not perceive the speed of a moving train while sitting inside the train. From the window of the moving train, the trees and buildings are seen moving. The trees or the houses are not moving, but seems so when viewed from the moving train. When the Earth is spinning, everything on the sky seems to be moving from East to West.
It is not just the Sun that rises in the East. The Moon, the stars and the visible planets rise from the east and spend around 12 hours in the sky before setting on the western horizon. If everything in the sky moves from East to West, it leaves us with two possibilities.
One, all the stars, planets, Moon, and the Sun are moving around us. Two, the Earth spins, creating an impression that everything in the sky is going around us. We can conclude that the Earth is spinning based on the moving train example.
Sun - The Source of Light for Earth
Sunrise is when darkness is lifted giving way to light. Sunset is when a place plunges into darkness. So we see that Sun is our main source of light, if not the only source.
Sun is a large ball-like object in the middle of the solar system. 8 smaller ball-like objects called planets go around the Sun on elliptical orbits. Earth is the third planet from the Sun in the solar system after Mercury and Venus.
Sun is not burning the way we understand the fires on Earth. The core of the Sun undergoes thermonuclear fusion releasing enormous amounts of energy. The light and the energy thus released keeps our planet warm enough for life forms to thrive.
One may wonder why thermonuclear fusion happens in Sun but not in Earth or other planets. The gravitational force of an object increases with its mass. Sun is approximately 333,000 times more massive than the Earth. The immense gravitational force experienced in its core causes nuclear fusion. Any object with 0.08 solar mass or more can support nuclear fusion and become a star. Earth or the other planets are much smaller and so nuclear fusion does not happen on its own.
The Phases of Moon
We know that Moon is lit by Sun. We can see the full lit side of Moon when it is opposite to Sun. This is called the full moon day. The dark side of Moon is visible when it is on the same side as Sun. This is called the New Moon day. After the new moon day, the lit portion of Moon keeps enlarging with every passing night until it becomes a full moon.
Given below are the terms denoting the various phases of the moon.
- New Moon - The Moon appears closer to sun in the sky and its dark side faces Earth.
- Waxing Crescent Moon - The appearance of a thin silver Moon after the New Moon is called the Waxing Crescent Moon.
- First Quarter Moon - Moon completes one quarter of its orbit around the earth and 50% of its surface is illuminated. Ever wondered why it is called a first quarter moon when half of its surface is lit? Read the earlier line and you may get the answer.
- Waxing Gibbous Moon - This is the growing phase when over 50% of its surface is lit.
- Full Moon - The full face of the Moon is lit. Sun and Moon are on either side of Earth for this to happen.
- Waning Gibbous Moon - This is the phase when the lit portion of Moon reduces in size but over 50% of its surface is lit.
- Third Quarter Moon - Moon completes 3/4th of its orbit around the Earth and 50% of its surface is lit.
- Waning Crescent Moon - The thin silver of the Moon appearing a day or two before the New Moon Day.
Lunar eclipse happens on a full moon day when Earth comes between Sun and the Moon. On most full moon days, Earth, Moon, and the Sun are not aligned in a straight line in the 3-dimensional space. When these three are aligned perfectly in a straight line, Earth casts its shadow on Moon causing the lunar eclipse. A lunar eclipse can be full or partial depending upon the Earth’s relative position.
This happens on a new moon day when Moon comes right in front of Sun to block the sunlight. Compared to Sun, the Moon is much smaller, but it is much closer to our planet. The Sun and Moon appear to be nearly of the same size when viewed from Earth. So the Moon can completely block the Sunlight if it comes right in between Earth and Sun.
If only a portion of the Moon blocks the Sun, then we get to glimpse a partial solar eclipse. If the full disk of the Sun is covered, it is called a total solar eclipse. For a region on Earth, a solar eclipse happens less often than a lunar eclipse.
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