A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Glossary of Astronomy Words

A

Angular size:
(also known as angular diameter, apparent diameter, or apparent size)

1. Apparent size is an angular measurement describing how large a sphere or circle appears from a given point of view.
2. It is the actual size of the object as seen by the observer from the point of observation.

(Image- credit: By Sriram.aeropsn - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=5745197)
 

Apparent motion:
Celestial objects appear to move in the sky because of motion of the observer. Their apparent motion is different from their actual motion. For example, the moon moved around the earth in approximately 27.3 days but it appears to move around the earth in approximately 25 hours because of the combination of the simultaneous motions of the earth and the moon.

Asteroids:

1. A small rocky body orbiting the sun.

2. Large numbers of these, ranging enormously in size, are found between the orbits of Mars and Jupiter, though some have more eccentric orbits.

Atmosphere:
The envelope of gases surrounding the earth or another planet. For atmosphere of the Earth visit: https://en.wikipedia.org/wiki/Atmosphere_of_Earth

B

C

Celestial object:
(astronomical object)

A naturally occurring physical entity, association, or structure that exists in the universe. (For example, a comet, a planet, a star, a galaxy)

Celestial sphere:
An abstract sphere, with an arbitrarily large radius, that is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere.

D

E

Eclipse:
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer.

Ecliptic:

1. Plane of the Earth’s orbit

2. Circular path on the celestial sphere that the Sun appears to follow over the course of a year.

Equinox:
The moment when the plane of Earth's equator passes through thecenter of the Sun's disk. In other words, it is the point in which the center of the visible sun is directly over the equator.

This occurs twice each year, around 20 March and 23 September. On equinox the daytime and night-time is of exactly same duration (12 hours each).

F

G

Gesture:
A movement of part of the body, especially a hand or the head, to express an idea or meaning.

Globe:
Globe is a model of our Earth. When we look at it, we can imagine how our Earth looks when it is seen from the space.

Globe has different sizes and shapes. But the overall features given on all the globes are same. Globe is made proportionate to the measure so that one can see actual features of the Earth showed on the globe.


(Image credit : by Kskhh, GEO Globe, CC-BY-SA-4.0 https://commons.wikimedia.org/wiki/File:GEO_Globe_10.jpg)

You must have studied that, globe has vertical lines knows as longitudes and horizontal lines known as latitudes.

The longest latitude of the earth i.e. diameter of the globe is called as equator. The globe is divided into two equal parts, northern hemisphere and southern hemisphere by the equator.

There are 2 latitudes which are at 23.5 degree from equator on both sides. The latitude which is on the North side of the equator is known as tropic of cancer and the one on the South side is known as tropic of capricorn.

The globe has central axis of rotation along which globe rotates. The axis of rotation of the globe is tilted by 23.5 degree. This is one of the main reason we experience different seasons on the Earth.

Globe has 7 continents and 5 oceans printed on it.
 

Gravitation:
It is a natural phenomenon by which all things with mass are attracted toward one another.
 

H

Hemisphere:
Half part of the sphere. For Earth, two terms are used.

1. Northern Hemisphere, the half that is north of the equator

2. Southern Hemisphere, the half that lies south of the equator

Horizon:
(skyline)
Aapparent line that separates earth from sky.

I

Illusion:
An instance of a wrong or misinterpreted perception of a sensory experience.
 

Impact Craters:
An approximately circular depression in the surface of a planet, moon, or other solid body in the solar System or elsewhere, formed when a smaller body hits on their surfaces with very high velocity.
 

J

K

Kelvin:
The kelvin is a unit of measure for temperature. Relation between degree and kelvin is Temp in K = Temp in Degree Celsius + 273.
 

L

Luminosity:

1. the intrinsic brightness of a celestial object.

2. It is the total amount of energy emitted per unit time by the celestial object.

Lunar eclipse:
A lunar eclipse occurs when the Moon passes directly behind the Earth into its shadow. This can occur only when the sun, Earth and moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, a lunar eclipse can occur only the night of a full moon.

M

N

Nashatras:
(Lunar mansion)
One of 27 sectors along the ecliptic. Their names are related to the most prominent group of stars in the respective sectors. It is used to chart position of the moon (and the planets).

Nuclear Fusion:

1. It is a reaction in which two or more atomic nuclei come close enough to form one or more different atomic nuclei and subatomic particles (neutrons and/or protons).

2. Nuclear fusion is the process of making a single heavy nucleus from two lighter nuclei.

O

Outer space (space):
The expanse that exists beyond the Earth’s atmosphere and between celestial bodies. Outer space is not completely empty - it is a mostly vacuum containing a low density of particles, predominantly a plasma of hydrogen and helium as well as electromagnetic radiation, magnetic fields, neutrinos, dust, and cosmic rays.

P

Phase of moon:
Phase of the moon is the shape of the illuminated (sunlit) portion of the Moon as seen by an observer on Earth.

Planet:
A celestial body moving in an orbit around a star.

Plasma:

1. an ionized state of matter similar to a gas

2. an ionized gas consisting of positive ions and free electrons in proportions resulting in no overall electric charge at very high temperatures (eg. in stars)

Q

R

Radius:
A radius of a circle (or sphere) is the length of the line from its center to perimeter.

Ratio:

1. a ratio is a relationship between two numbers. It indicates how many times the first number contains the second.

2. A ratio is a comparison of two numbers by division.

Reflection:
Reflection is the change in direction of a ray (or a wave) at a boundary between two different media, so that the wave moves back into the medium it came from.

Revolution:
Movement of one body around another in a curved path (orbit)

Rotation:

1. a circular movement of a two dimensional object (e.g. a disc) around a point of rotation.

2. A three-dimensional object can always be rotated around an infinite number of imaginary lines called rotation axis

S

Satellite:
Satellite is an object which revolves around planet. There are two type of satellites

1. A natural satellite or moon is an astronomical body that orbits a planet.

2. Artificial satellite are man made object which are placed intentionally into the space and which revolves around different planets.

Self luminous:
Having in itself the property of emitting light

Sidereal day:
Time taken by the earth to rotate on its axis relative to the stars (not the sun). It is almost four minutes shorter than the solar day because of the earth's orbital motion

Sidereal month:
It is the time taken by the Moon to return to a similar position among the fixed (background) stars. Time for sidereal month is 27.322 days

Sky:
Everything that lies above the surface of the Earth, including the atmosphere and outer space.

Solar eclipse:
A solar eclipse occurs when the Moon passes in between the Sun and Earth, and the Moon fully or partially blocks the Sun. This can happen only at new moon.

Solar day (Day):
Solar day is a time required by the Earth to rotate around itself measured relative to sun. It is the time between two consecutive sunrises.

Solstice:
Event occurring when the Sun’s path appears to either towards most north or most south. Two solstices occur annually, on about 21 June and 21 December. These are longest or shortest days (depending upon in which hemisphere you are).

The word solstice is derived from the Latin sol ("sun") and sistere ("to stand still"), because at the solstices, the Sun's declination "stands still" or stops at a northern or southern limit before reversing direction.

Star:
A luminous sphere of plasma held together by its own gravity

Synchronous:
A process or an activity happening at the same time or rate.

Synodic month:
It is the time between two successive new moons or full moons. Time for sidereal month is 29.531 days.

T

Tangent:
A straight line that touches a curve at a point, but if extended does not cross it at that point.

Tidal locking:
Tidal locking is a situation when an object's orbital period (rotation) matches its rotational period (revolution).

Tithi:
Lunar day. Time period between two consecutive moonrises in a particular location on Earth.

U

V

Vacuum:
Space devoid of matter

W

Weight:

1. a body's relative mass or the quantity of matter contained by it, giving rise to a downward force; the heaviness of a person or thing.

2. the weight of an object is usually taken to be the force on the object due to gravity.

X

Y

Z

Zodiac (or astrological) signs:
In Western astrology, astrological signs are the twelve 30° sectors of the ecliptic, starting at the vernal equinox.