Astronomy , chapter 1
Solstice
("Sol"-sun, and "stare"-stand) either of the two times a year when the sun is at its greatest distance from the celestial equator.
Penumbra
(i) Portion of the shadow cast by an eclipsing object in which the eclipse is seen as partial. (ii) The outer region of a sunspot, surrounding the umbra, which is not as dark, and not as cool as the central region
Scientific notion
expressing large and small numbers using power of 10 notation.
Sidereal Time
is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky. Briefly, sidereal time is a "time scale that is based on the Earth's rate of rotation measured relative to the fixed stars"[1] rather than the Sun.Also known as local sidereal time. It is the time since the Vernal Equinox was on the meridian of the observer. It can also be expressed as the Hour Angle of the Vernal Equinox.
Sidereal period
the period of time that a planet takes to revolve around the sun with respect to the stars. It is the motion from one point on the planet's orbit back to the same point.
Scientific Method
the set of rules used to guide science, based on the idea that scientific laws be continually tested, and modified or replaced if found wrong.
Lunar phase
1.New moon(all but invisible in the sky) 2.Moon appears to wax(grow) 3.crescent 4. half of the lunar disk can be seen quarter Moon 6.during the next week , the Moon passes through gibbous phase, in two weeks full moon is visible. during the next two weeks Moon wanes (shrinks) , passing in turn through the gibbous , quarter , cresent phases and eventually becoming new again.
Corona
A corona (Latin, 'crown') is an aura of plasma that surrounds the sun and other celestial bodies. The Sun's corona extends millions of kilometres into space and is most easily seen during a total solar eclipse, but it is also observable with a coronagraph. The word "corona" is a Latin word meaning "crown", from the Ancient Greek κορώνη (korōnē, "garland, wreath").
Sidereal
A generic term meaning "with respect to the stars".
Constellations
A human grouping of stars in the night sky into a recognizable pattern.
Synodic day
A synodic day is the period it takes for a planet to rotate once in relation to the body it is orbiting. For Earth, the synodic day is known as a solar day, and is about 24 hours long. The synodic day is distinguished from the sidereal day, which is one complete rotation in relation to distant stars. A synodic day may be "sunrise to sunrise'" whereas a sidereal day can be from the rise of any star to the rise of the same star on the next day. These two quantities are not equal because of the body's movement around its parent.
Celestial equator
An imaginary projection of the geographic equator onto the celestial sphere. Declination is measured north and south from the celestial equator. The celestial equator is zero declination.
celestial sphere
An imaginary sphere surrounding and centered on the earth. It is very much larger than the earth. The vast majority of celestial objects are so far from the earth, that they can be treated mathematically as if they were moving on this imaginary sphere. The stars are imagined to be fixed on the sphere which appears to rotate around the earth once each day. The sun, moon and planets are imagined to move on the sphere with respect to the stars.
Revolving
Any circular motion around a point outside the object in contrast to rotation which is circular motion about a point at the center of the object. See revolution and rotation.
Apogee
Apogee and perigee refer to the distance from the Earth to the moon. Apogee is the farthest point from the earth
Phase
Appearance of the sunlit face of the Moon or a planet at different points along its orbit.The moon's appearance undergoes a regular cycle of changes,or phases, taking roughly 29.5 days to complete.
Solar eclipse
Celestial event during which the new Moon passes directly between the Earth and Sun, temporarily blocking the Sun's light.
Umbra
Central region of the shadow cast by an eclipsing body. (ii) The central region of a sunspot, which is its darker and cooler part.
Seasons
Changes in average temperature and length of day that result from the tilt of Earth's (or any planet's) axis with respect to the plane of its orbit.
Eclipse
Event during which one body passes in front of another , so that the light from the occulted body is blocked. From time to time but only at new or full moon the Sun and the Moon line up precisely.
Galaxy
Gravitationally bound collection of a large number of stars. The Sun is a star in the Milky Way Galaxy.
Latitude
In an equatorial coordinate system, latitude is the angle measured from the equator directly towards the north pole or the south pole. This measurement will follow the meridian through the object of interest.
Precession
In astronomy, "precession" refers to any of several slow changes in an astronomical body's rotational or orbital parameters, and especially to Earth's precession of the equinoxes.
Axial tilt
In astronomy, known as obliquity, is the angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane.[1] It differs from orbital inclination.
Aphelion
It is the opposite of aphelion, which is the point in the orbit where the celestial body is farthest from the Sun.
Triangulation
Method of determining distance based on the principles of geometry. A distant object is sighted from two well-separated locations. The distance between the two locations and the angle between the line joining them and the line to the distant object are all that are necessary to ascertain the object's distance.
Revolution
Orbital motion of one body around another, such as Earth around the Sun.
celestial coordinates
Pair of quantities-right ascension and declination-similar to longitude and latitude on Earth, used to pinpoint locations of objects on the celestial sphere.
Perigee
Perigee is the closest point to the earth and it is in this stage that the moon appears larger. Looking at the moon in the sky without anything to compare it to, you wouldn't notice any size difference.
Summer solstice
Point on the ecliptic where the Sun is at its northernmost point above the celestial equator, occurring on or near June 21.
Winter solstice
Point on the ecliptic where the Sun is at its southernmost point below the celestial equator, occurring on or near December 21.
Annular eclipse
Solar eclipse occurring at a time when the Moon is far enough away from the Earth that it fails to cover the disk of the Sun completely, leaving a ring of sunlight visible around its edge.
Difference between solar and sidereal days
Solar is 24 hour, sidereal is less. The Earth moves in two ways simultaneously. It rotates on its axis at the same time revolving around the Sun. Solar day is 3.9 min longer than sidereal
Rotation
Spinning motion of a body about an axis. The Earth rotates in one day
Solar System
The Sun and all the bodies that orbit it-Mercury, Venus Of those objects that orbit the Sun directly, the largest eight are the planets, with the remainder being significantly smaller objects, such as dwarf planets and small Solar System bodies such as comets and asteroids.Of those that orbit the Sun indirectly, two are larger than the smallest planet.
Parallax
The apparent motion of a relatively close object with respect to a more distant background as the location of the observer changes.
Ecliptic
The apparent path of the Sun, relative to the stars on the celestial sphere, over the course of a year. It is the orbit of the earth extended to the celestial sphere.
Astronomical Unit
The astronomical unit (symbol au,[1][2] AU[3][4][5] or ua[6]) is a unit of length, roughly the distance from the Earth to the Sun. However, that distance varies as the Earth orbits the Sun, from a maximum (aphelion) to a minimum (perihelion) and back again once a year.????/
Equinox
The direction in space where the celestial equator crosses the ecliptic. There are two such directions.
Vernal Equinox
The direction where the sun traveling on the ecliptic would cross the equator from south to north is called the Vernal (or Spring) Equinox.The Vernal Equinox occurs on about March 21 each year
Baseline
The distance between two observing locations used for the purposes of triangulation measurements. The larger the baseline, the better the resolution attainable.
Light Year
The distance that light, moving at a constant speed of 300,000 km/sec, travels in one year. One light-year is about 10 trillion kilometers (1013 km).
Horizon
The horizon or skyline is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not.
Perihelion
The perihelion is the point in the orbit of a planet, minor planet, or comet, where it is nearest to the Sun.
Solar day
The period of time between the instant when the Sun is highest in the south (north if you are in the southern hemisphere) (i.e. at noon) to the next time it is in that position. The highest point occurs when the sun crosses the observer's local meridian. Period from noon to the next, the 24-hour solar day
Tropical year
The time interval between one vernal equinox and the next.A tropical year (also known as a solar year), for general purposes, is the time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice.
Sidereal day
The time needed for a star on the celestial sphere to make one complete rotation in the sky. It is also defined as the time it takes the earth to rotate once with respect to the stars. A day measured by stars.
Sidereal year
The time required for the constellations to complete once cycle around the sky and return to their starting points, as seen from a given point on Earth. This can also be viewed as the time it takes the Earth to complete one circuit of its orbit with respect to the stars.
Universe
The totality of all space and time. All that is, has been and will be. The visible Universe is a subset of the Universe.
Zodiac
The twelve constellations through which the Sun moves as it follows its path on the ecliptic.
Celestial pole
Those two points in the sky that are directly above the north (north celestial pole) and south (south celestial pole) geographic poles of the Earth. As the Earth turns, the sky appears to rotate about the celestial poles.
Eclipse year
Time interval between successive orbital configurations in which the line of nodes of the Moon's orbit points toward the Sun.
Synodic month
Time required for the Moon to complete a full cycle of phases. This is usually measured from new moon to the next new moon and take about 29.5 solar days.The synodic month is a little longer than sidereal month
Sidereal month
Time required for the Moon to complete one trip around the celestial sphere. If the Moon is near in direction to a distant star, it will take it one sidereal month to return to the same position with respect to the direction to the star. The length of the sidereal month is about 27.3 solar days.
Eclipse season
Times of the year when the Moon lies in the same plane as the Earth and Sun, so that eclipses are possible.
Degree
Unit of angular measure.There are 360 degree in one complete circle.
Total and partial eclipse
Usually alignment of the Sun ,Earth and the Moon is imperfect , so the shadow never completely covers the Moon. Occasionally, however the entire lunar surface is obscured in a total lunar eclipse. When the shadow is not complete it is partial eclipse.
Lunar eclipse
When the Sun and the Moon are in exactly opposite direction, Earth shadow sweeps across the Moon, blocking the Sun's light Celestial event during which the moon passes through the shadow of the Earth, temporarily darkening its surface.
Autumnal Equinox
Where the sun would cross the equator from north to south is called the Autumnal (or Fall) Equinox. The dates on which these events are also called the equinoxes. Autumnal Equinox occurs on or about September 23. The name (equinox) is Latin and means "equal night". On the date of an equinox, the number of hours of daylingt equals the number of hours of night.
Zenith
Zenith refers to an imaginary point directly "above" a particular location, on the imaginary celestial sphere.highest point,
Equator
an imaginary line on the Earth's surface equidistant from the North Pole and South Pole, dividing the Earth into the Northern Hemisphere and Southern Hemisphere.
Diurnal motion
apparent daily motion of stars around the Earth, or more precisely around the two celestial poles.
