Albedo:
In astronomy, the term albedo refers to the brightness of an object in space. Derived from Latin, albedo means “whiteness” (albus= “white”). Albedo is measured on a scale from 0 to 1, where 0 refers to an object that is completely dark, i.e., it does not reflect any light. 1 on the scale refers to a perfectly reflective object. The Moon has an Albedo of 0.12, while Earth’s average albedo is 0.3.
Altitude (elevation):
Altitude or elevation is the vertical angle an object makes with the horizon.
Annular Solar Eclipse:
An annular solar eclipse happens when the Moon covers the Sun’s center, leaving the Sun’s visible outer edges to form a “ring of fire” or annulus around the Moon.
Antumbra:
The antumbra is the lighter part of a shadow that forms at a certain distance from the object casting the shadow. It is involved in annular solar eclipses and planet transits.
Asterism
A pattern of stars recognizable to observers from Earth. Asterisms may or may not be part of a constellation. The Big Dipper is one of the most well-known asterisms. Its stars belong to the constellation Ursa Major.
Asteroid
These are found between the orbits of Mars and Jupiter. These are celestial bodies with sizes ranging from a few meters to hundreds of the kilometres of diameter, revolving around the sun. They have originated by the disintegration of the planets.
Astronomical Season
Astronomers and scientists use the dates of equinoxes and solstices to mark the beginning and end of seasons in a year. In the Northern Hemisphere, the four astronomical seasons are:
Spring: March equinox to June solstice.
Summer: June solstice to September equinox.
Fall (autumn): September equinox to December solstice.
Winter: December solstice to March equinox.
Astronomical Twilight
Astronomical twilight is the darkest of the three twilight phases. It is the earliest stage of dawn in the morning and the last stage of dusk in the evening.
Astronomical Unit
The average distance between Earth and the Sun, 1.5 x 108 km
Atmospheric Phenomena
Atmospheric phenomena occur when light, usually from a natural source, but sometimes from artificial sources, is reflected or refracted as it passes through the atmosphere, for example by air molecules, ice crystals, or different types of particles.
Aurora
The glowing light from solar particles interact with Earth’s magnetic field is called aurora
Aurora Borealis and Aurora Australis
An aurora is a natural phenomenon that creates bright and colorful light displays in the sky. In the Arctic Circle, they are known as aurora borealis or northern lights; in the Antarctic Circle, they are called aurora australis or southern lights.
Axial Tilt – Obliquity
Earth’s axis is tilted by about 23.4 degrees to the perpendicular to the orbital plane. Earth rotates the Sun at a slant. This means that different amounts of sunlight reach the Northern and Southern Hemispheres throughout the year. This is the reason we have seasons on Earth.
Axis
An axis in astronomy refers to the (imaginary) line that an object, usually a planet, rotates around. Earth’s rotational axis is an imaginary straight line that runs through the North and South Poles
Azimuth
An object’s cardinal direction, such as north, east, south, or west
Black Hole
A Neutron Star remains contracting and, thus, mass in a large quantity, concentrates on one point. Such body with high density is called Black Hole. It does not allow anything to escape, including the light due to which it can be seen. It is John Wheeler who propounded the concept of Black Hole
Black Moon
It can be the third New Moon in an astronomical season with four New Moons or the second New Moon in the same calendar month.
Two Definitions of Blue Moon
Seasonal Blue Moon = The third Full Moon in an astronomical season with four Full Moons (versus the usual three).
Monthly Blue Moon = The second Full Moon in a month with two Full Moons.
If the moon actually looks blue, it’s caused by a rare type of dust in the atmosphere.
Celestial Equator
The celestial equator is the projection of Earth’s equator onto the celestial sphere. From our perspective, it is the part of the sky directly above the equator.
Celestial Horizon
The imaginary horizontal line separating the two hemispheres of the celestial sphere is called the celestial horizon.
Celestial Pole
The celestial poles are imaginary lines that trace Earth’s rotation axis in space. From our perspective, they are the points in the sky directly above the North Pole and the South Pole. Because of this, objects that lie on the celestial pole do not seem to move at all, while all other objects, mostly stars, seem to move in a circle around the pole.
Celestial Sphere
The celestial sphere is an imaginary sphere that extends infinitely into space with Earth at its center. It is the backdrop the horizontal coordinate system uses to map the sky and describe the positions of its objects.
Circumpolar Stars
Circumpolar stars never set or go below the horizon for observers from specific latitudes. They are visible to observers from these latitudes throughout the year because of their proximity to the celestial pole. Circumpolar objects lie within the circumpolar circle, and stars circumpolar to latitudes in the Northern Hemisphere are not visible in the Southern Hemisphere, and vice versa.
Civil Twilight
Civil twilight is the brightest of the three twilight phases. The Sun is just below the horizon, so there is generally enough natural light to carry out most outdoor activities.
Comet
One of the small, icy bodies that orbit the sun that make tails of gas and dust when they get close to the sun. There are the bodies composed of dust, ice and gases, which come from the colder and darker areas, away from the sun. They go around the sun in large and irregular orbits. While moving in their orbits, when they come very close to the sun, they start glowing with a bright gaseous tail always pointing away from the sun. Many a times, comets are visible to the naked eye and present a very spectacular sight. Comet Halley, discovered by Edmund Halley, returns after every 76 years
Constellation
In the present day, a constellation is a two-dimensional area in space as defined by the International Astronomical Union (IAU). Today, there are 88 officially recognized constellations. This is different from the historical understanding of a constellation dominantly used in astrology which is defined as a group of stars easily identifiable by a pattern to observers on Earth.
Crust
The land and oceans at the Earth’s surface lie on an outer layer of cool, hard rock called the crust.
Core
At the very centre of the Earth is a super-hot ball of iron called the core
Dawn
Dawn is the transition from night to day as the sky gets brighter. Scientists distinguish between three definitions of dawn
civil, nautical, and astronomical dawn. Each one is a specific moment in time, based on the solar elevation angle.
Dusk
Dusk generally refers to the transition from day to night. Scientists distinguish between three definitions of dusk
civil, nautical, and astronomical dusk. Each one is a specific moment in time, based on the solar elevation angle.
Diamond Ring
Just before the moon completely covers the sun, mountains on the moon allow a bit of light to peek through creating this beautiful diamond ring effect at right
Eight planets
Solar System consists of eight “planets” Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Ecliptic
The path the Sun seems to follow in the sky
Earthshine
Earthshine is a dull glow which sometimes lights up the unlit part of the Moon. It usually occurs a few days before and after a New Moon when sunlight reflects off Earth’s surface and illuminates the portion of the Moon’s surface which is not lit up by direct sunlight.
Eclipses
A solar eclipse happens when the New Moon moves between Earth and the Sun. A lunar eclipse occurs when Earth casts a shadow onto the Full Moon.
Ecliptic — Ecliptic Plane – Orbital Plane
The ecliptic plane, also called orbital plane, is the imaginary line that traces the Sun’s apparent path in the sky. In other words, it is the projection of our planet’s orbit into the celestial sphere. Any constellations on this line are known as zodiacal or zodiac constellations.
Earth’s Axial Tilt or Obliquity
When an object the size of Mars crashed into the newly formed planet Earth around 4.5 billion years ago, it knocked our planet over and left it tilted at an angle.
Elevation (altitude)
Elevation or altitude is the vertical angle an object makes with the horizon.
Equinox
There are two equinoxes every year – in March and September – when the Sun is directly above the equator and the length of night and day are nearly equal.
False Dawn
False dawn or zodiacal light is a rare optical phenomenon that occurs around sunset and sunrise, usually during early spring and late fall.
Full Moon
The Full Moon is the Moon phase when the entire face of the Moon is lit up.
Great Circle
A great circle is any circle that is formed by a plane that passes through the center of Earth. The equator and the circles created by the meridians form great circles.
Halos
Halos are atmospheric phenomena created by light which is reflected or refracted by ice crystals in the atmosphere.
Horizontal Coordinate System
The horizontal coordinate system, also known as the Alt/Az system, is a method for describing the exact position of objects in the sky, such as planets, the Sun, or the Moon.
Light Pillars
Light Pillars are an atmospheric phenomenon created by light which is reflected by ice crystals in the atmosphere.
Light Year
A light year (also spelled light-year or lightyear) is a unit of distance and is defined as the distance traveled by light in a vacuum during a Julian year. In astronomy, a Julian year (symbol
a) is a time unit defined as exactly 365.25 days of 86,400 seconds each. The distance is approximately 9.5 trillion kilometers or 6 trillion miles.
Local Midnight
Local midnight typically occurs when the Sun crosses the meridian below the horizon. In areas where there is midnight Sun, local midnight is when the Sun is at its lowest point of the night.
Lunar Apogee
The point of the Moon’s orbit farthest from Earth is called apogee.
Lunar Month
A lunar month is the time it takes the Moon to pass through all of the Moon phases, measured from one New Moon to the next. A lunar month is also known as a lunation, while the astronomical term for this period is a synodic month.
Lunar Perigee
The point of the Moon’s orbit closest to Earth is called perigee.
Lunation
A lunation is the time it takes the Moon to pass through all of the Moon phases, measured from a New Moon to the next New Moon. A lunation is also known as a lunar month, while the astronomical term for this period is a synodic month. Lunations are numbered in several different systems; the most common one is the Brown lunation number system, which we use our Moon phase pages.
Magnetic Declination
The difference between true north and magnetic north is called magnetic declination or magnetic variation.
Meridian
A meridian is an imaginary line that runs from the North Pole to the South Pole, connecting all locations sharing the same longitude. The moment when the Sun or the Moon crosses a location’s meridian marks the instant when they reach the highest position in the sky, appearing either due south, due north, or directly overhead. For the Sun, it is the moment of solar noon.
Meteor
When a meteoroid enters Earth’s atmosphere, it starts to glow as it collides with air molecules in the upper atmosphere. The flash of light this generates is called a meteor, a shooting star, or a falling star. Meteors are the celestial bodies composed of dust and gases. After coming under the influence of earth’s gravity, they move with a great velocity towards the earth. But, due to collision with the particles of the atmosphere they burn and get converted into ash.
Meteor Shower
A meteor shower is when an unusual amount of meteors—or shooting stars—flash across the night sky over a period, usually a few days or weeks.
Meteorite
A meteorite is the name for a space rock or meteoroid which has survived falling through the atmosphere and has landed on Earth.
Meteoroid
Meteors which are large in size and do not burn completely and reach the surface of the earth are called meteorites. A meteoroid is a block of matter made up of dust particles or fragments from a comet or an asteroid. Meteoroids become meteors, also called shooting stars, when they enter Earth’s atmosphere burning a trail of dust and fire which is visible from Earth as a flash of light in the sky.
Micromoon
When a Full Moon or a New Moon occurs around apogee, which is the point on the orbit farthest from Earth, it’s called a Micromoon, Minimoon, or Apogee Moon. When there is a Full or New Moon around perigee, it is called a Supermoon.
Midnight Sun
Polar Day:Midnight Sun is when at least a part of the Sun’s disk is visible above the horizon 24 hours of the day. The scientific name for midnight Sun is polar day, and the opposite is polar night.
Moon Phase
The sunlight that reflects onto the Moon’s surface we call a Moon phase. How much of that light we can see from our point of view on Earth varies every day. The lunar month is generally divided into four primary and four intermediate Moon phases
New Moon, Waxing Crescent Moon, First Quarter Moon, Waxing Gibbous Moon, Full Moon, Waning Gibbous Moon, Third Quarter Moon and Waning Crescent Moon
Moonbows
Moonbows or lunar rainbows are rare natural atmospheric phenomena that occur when the Moon’s light is reflected and refracted off water droplets in the air.
Moonrise and Moonset
Moonrise is defined as the moment the upper edge of the Moon becomes visible above the horizon. The Moon sets as the upper edge disappears below the horizon. Sometimes, the Moon is not visible even if it is above the horizon. This is the case, for example, during the New Moon and when the sky is not clear.
Magma
Inside the centre of the Earth there is red-hot, liquid rock. This rock is called magma.
Mantle
The hot magma below rises and sinks slowly in a layer called the mantle.
Neutron Star
If the mass of the star is much more than the mass of the sun then it became Neutron Star.
Nautical Twilight
Nautical twilight is the second twilight phase. Both the horizon and the brighter stars are usually visible at this time, making it possible to navigate at sea.
Northern Lights — Aurora Borealis
An aurora is a natural phenomenon that creates bright and colorful light displays in the sky. In the Arctic Circle, they are known as aurora borealis or northern lights.
Obliquity – Axial tilt
Earth’s axis is tilted about 23.4 degrees to the perpendicular to the orbital plane. Our planet rotates the Sun at a slant. This means that different amounts of sunlight reach the Northern and Southern Hemispheres throughout the year. This is the reason we have seasons on Earth.
Orbit
An orbit is the path of an object around a point or another object in space. In general, this path is repeatedly followed by the object, though, in some cases, different celestial forces like gravitation can change its orbit. An object in orbit is called a satellite. Orbits are formed due to two opposing forces—the orbiting object’s momentum and the force of gravity that pulls it towards the object it is orbiting. These two forces have to balance each other for an orbit to be sustained.
Parhelic Circle
A parhelic circle is a rare optical atmospheric phenomenon.
Partial Lunar Eclipse
A partial lunar eclipse occurs when Earth moves between the Sun and Moon, but the three bodies do not form a perfectly straight line in space. When this happens, only part of the Moon’s surface is covered by the darkest part of the shadow cast by Earth, the umbra.
Partial Solar Eclipse
During a partial solar eclipse, only part of the solar disk is covered by the New Moon.
Penumbra
The penumbra is the lighter outer part of a shadow. The Moon’s penumbra causes partial solar eclipses, and Earth’s penumbra is involved in penumbral lunar eclipses.
Penumbral Lunar Eclipse
A penumbral lunar eclipse occurs when the Sun, Earth, and the Moon align in an almost straight line. When this happens, Earth covers all or part of the Moon with the outer part of its shadow, also known as the penumbra. Since the penumbra is much fainter than the dark core of the Earth’s shadow, the umbra, a penumbral eclipse of the Moon is often difficult to tell apart from a normal Full Moon.
Pink Moon
The Full Moon in April is the Pink Moon, from the pink flowers – phlox – that bloom in the early spring.
Plates
The earth’s surface is cracked into large pieces, called plates, which fit together like an enormous jigsaw. There are nine large plates and several smaller ones.
Planetary Transit
A planetary transit occurs when a planet passes in front of the Sun. It is then visible from Earth as a tiny black dot silhouetted against the Sun’s disk. The only two planets that can be seen transiting the Sun from Earth are Mercury and Venus, because they are the only planets inside Earth’s orbit.
Polar Day — Midnight Sun
Polar day or Midnight Sun is when at least a part of the Sun’s disk is visible above the horizon 24 hours of the day. The scientific name for midnight Sun is polar day, and the opposite is polar night.
Polar Night
Polar night happens when the entire Sun remains below the horizon all day. It only happens within the polar circle, and the opposite of polar night is midnight Sun or polar day.
Reflection
Reflection occurs when light bounces off the surface of an object.
Refraction
Refraction is the bending of light as it moves from one substance to another. For example, it happens when sunlight enters Earth’s atmosphere.
Shooting Star
A Shooting star is a popular term for a meteor, which is a flash of light generated when a meteoroid enters Earth’s atmosphere.
Solar Day
Modern timekeeping defines a day as the sum of 24 hours – but that is not quite correct. In solar time, most days are a little longer than 24 hours, the time it takes from one solar noon to the next.
Solar Noon
Solar noon occurs when the Sun crosses a location’s meridian and reaches its highest position of the day. In most locations, it doesn’t happen at 12 o’clock. Find Sun times worldwide.
Solar Time
A sundial shows the true or apparent solar time. Because Earth’s rotation is not constant, solar days vary slightly in length. This means that the speed of true solar time is not constant. Mean Solar Time is based on the length of a mean or average solar day, which is 24 hours long. It moves at a constant speed.
Solar Wind
Solar wind is part of space weather. It is a continuous stream of highly energized particles – mostly electrons and protons – that flow out from the Sun through space at very high speeds and high temperature. Northern lights are caused by high solar wind activity.
Solstice
Solstices happen twice a year — in June and December. The June solstice is around June 21, when the Sun is directly overhead the Tropic of Cancer. The December solstice takes place around December 21. On this day, the Sun is precisely over the Tropic of Capricorn. The winter solstice is the shortest day of the year, while the summer solstice is the longest day of the year. Solstices are opposite on either side of the equator.
Southern Lights — Aurora Australis
An aurora is a natural phenomenon that creates bright and colorful light displays in the sky. In the Antarctic Circle, they are called aurora australis or southern lights.
Summer Solstice
The summer solstice is the longest day of the year. Solstices are opposite on either side of the equator; the summer solstice in the Northern Hemisphere is the June solstice, while in the Southern Hemisphere, it is the December solstice.
Sundogs
Sundogs are an atmospheric phenomenon created by light which is reflected or refracted by ice crystals in the atmosphere.
Sunspots
Sunspots are areas where the cooler areas show up darker than the surrounding surface. Some sunspots are larger than the diameter of the Earth and can reach up to 20,000 km in diameter.
Solar flares
Solar flares are jets of particles that burst from the sun and can disrupt satellite communications and knock out electricity on Earth. Solar flares can sometimes leave the sun and zoom towards Earth. When the high speed particles from the sun contact the Earth’s magnetic field it produces a lighting effect known as the aurora.
Supernova
The helium starts converting into carbon and carbon into heavy metal like iron in the core. This results in a massive explosion in the which is known as supernova
Sunrise
Sunrise is defined as the moment the upper edge of the solar disk—called the upper limb—becomes visible above the horizon.
Sunset
As the upper edge of the solar disk—called the upper limb—disappears below the horizon, the Sun has set.
Supermoon
When a Full Moon or a New Moon occurs around perigee, which is the point on the orbit closest to Earth, it’s called a Supermoon. When a Full Moon takes place when the Moon is near its closest approach to Earth, it is called a Super Full Moon. When there is a New Moon around the closest point to Earth, it is known as a Super New Moon.
Synodic Month
A synodic month is the time it takes the Moon to pass through all of the Moon phases, measured from a New Moon to the next New Moon. It is also known as a lunar month or a lunation.
Tides
The gravitational pull of the Moon and the Sun makes the water in the oceans bulge, causing a continuous change between high and low tide.
Total Solar Eclipse
During a total eclipse of the Sun, the Moon covers the entire disk of the Sun. A total solar eclipse is only total within the path of totality and only for a short while. Most of the time and in most places, it is visible as a partial solar eclipse.
Tropical Year
A tropical year is the time it takes Earth to complete a full orbit around the Sun. Its duration varies from year to year. Also known as a solar year, an astronomical year, or an equinoctial year, it is, on average, approximately 365 days, 5 hours, 48 minutes and 45 seconds long (365.24219 days).
Twilight
Twilight is the time between day and night when there is light outside, but the Sun is below the horizon. There are three degrees of twilight civil, nautical, and astronomical twilight.
Umbra
The umbra is the darkest portion of a shadow. The Moon’s umbra causes total solar eclipses, while Earth’s umbra sometimes creates total and partial lunar eclipses.
Zenith
In celestial coordinate systems, the location straight above you is called zenith while the point exactly below you is referred to as nadir.
Zodiacal Constellations
Zodiacal constellations are the constellations located within Earth’s ecliptic. There are 13, not 12, zodiacal constellations. The 13th constellation is Ophiuchus.
Zodiacal Light
Zodiacal lights or false dawn is a rare optical phenomenon that occurs around sunset and sunrise, usually during early spring and late fall.
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