Chapter 1 : Introduction to Astronomy
Astronomy is the study of the universe.
It is a natural science that deals with the study of celestial objects (such as moons, planets, stars, nebulae, and galaxies).
Topics covered in this snack-sized chapter:
The Milky Way is the galaxy that contains our Solar System.
Picture of the Milky Way is shown below:
- The nature of the universe itself.
To find answers to below questions:
- Why there are days and nights, and seasons on the Earth?
- Why do stars change position at night in the sky?
To understand Astronomy, it is necessary to understand the scale.
- Is the Universe infinite in scope and size?
- Earth is a tiny object as compared to the Sun.
- The Sun is one star amongst over 200 billion
stars gravitationally bound together to make the Milky Way Galaxy.
A Light Year is the distance traveled by light in one year.
One light year = 9.46 × 1015
One light year = 9,460,730,472,580.8 km
A group of fixed stars is known as Constellation.
Human imagination can convert a group of stars into a known figure.
Below picture is showing one such conversion:
They are designated in most cases by the name of:
The Celestial Sphere is an imaginary sphere of large radius, concentric with the Earth and rotating upon the same axis.
All objects in the sky can be thought of as projected upon the celestial sphere.
Equator and the poles are projected upward from Earth's equator and poles and are called the celestial equator and the celestial poles.
- Age of the universe is known to be around billion years.
The celestial sphere is a very practical tool for the positional astronomy.
The entire sky appears to turn around imaginary points in the northern and southern sky once in 24 hours.
Except for a few nearby galaxies, all the galaxies are moving away from us.
- The recession speed of a galaxy is proportional to its distance from us.
- A galaxy that is twice as far away is moving twice as fast.
This implies that on large scales, all the galaxies are moving away from each other.
There are many familiar astronomical cycles:
The apparent motion of the Sun defines the day.
The rotation of the Earth affects:
At night, the stars move in fixed patterns.
The stars rotate about a fixed point in the northern sky.
We have the Sun rising and setting, and the stars moving in fixed patterns about a point in the sky.
The Earth is spinning on its axis.
The Earth and the other planets revolve around the Sun.
During the course of 27 days, the Earth moves around a substantial part of its orbit about the Sun.
It takes extra 2 days for the Moon to “catch up” with the Sun.
The full Moon always rises just after sunset.
The crescent Moon always points towards the Sun.
A crescent Moon sets shortly after sunset, or rises just before sunrise.
- Every day, the Sun rises in the east and sets in the west.
The Moon is illuminated by reflecting the sunlight.
The lit side of the Moon always faces the Sun.
Because of the motion of the Moon relative to the Sun, we see different amounts of lit and dark sides over the course of a month called the Lunar Cycle.