| Term | Definition |
|---|
| Celestial Coordinate Systems | Galactic, Horizontal/Horizon(Alt-Azimuth), Equatorial |
| Right Ascension | analogous to terrestrial longitude, designated as RA. It is measured with respect to the Vernal Equinox which has a RA of 0 hour. It is the angular distance (in uits of time or angular measure) eastward from the Vernal Equinox, as measured along the celestial Equator. The Celestial Sphere is divided into 24-hour lines of RA, each hour line is measure of 15 degrees in angular measure, therefore each degree is equal to 4 minutes of time measure. EX: Betelgeuse 7 deg. 24 minuites * 5 hours 55 minutes (7°24" * 5hr55") |
| Declination | Analogous to terrestrial latitude, designated as DEC. It is measured perpendicular from the Celestial Equator to the NCP or the SCP along a meridian line or, as it is called on the celestial sphere, an Hour Circle. It is the angular distance in degrees north or south of the celestial Equator that has a DEC of 0 degrees. North of the Celestial Equator, the DEC is positive, south of the Celestial Equator, it is negative. The NCP has a DEC of 90 degrees north or +90 degrees, while the SCP has a DEC of 90 degrees south or -90 degrees. EX: Betelgeuse 7 deg. 24 minuites * 5 hours 55 minutes (7°24" * 5hr55") |
| Celestial Meridian | the imaginary circle on the celestial sphere passing through the observer's zenith and the north celestial pole, then intersecting the horizon at the north and south points |
| Celestial Equator | The projection of Earth's equator onto the Celestial Sphere. It is 90 degrees from the Celestial poles in every direction. As result of the Earth's axial tilt, it is inclined by ~23.5° with respect to the ecliptic plane. |
| Ecliptic | The apparent path of the Sun traces out in the sky with respect to the stars in the course of a year. This is actually Earth's path round the Sun. The ecliptic is at ~23.5° to the equator. |
| Vernal Equinox | The point when the Sun crosses the Celestial Equator. this is therefore the intersection of the Celestial Equator and the Ecliptic. This occurs on the first day of spring, usually on March 21. also known as the First Point of Aries. RA=0 hour. |
| Autumnal Equinox | The point when the Sun crosses the Celestial Equator second time of the year. The intersection of the Celestial Equator and the Ecliptic, occurs on the first day of autumn, usually on Sept 21. RA=12 hour. |
| North Celestial Pole | An extension of the Earth's rotational axis intersects the Celestial Sphere and the extension of Earth's North Pole, designated by the Northern Pole Star, currently Polaris. |
| South Celestial Pole | An extension of the Earth's rotational axis intersects the Celestial Sphere and the extension of Earth's South Pole, designated by the Southern Pole Star, currently Sigma Octantis (5.5 apparent magnitude.) |
| North Star | Its the prominent pole star that lies closest in the sky to the north celestial pole and which appears (approximately) directly overhead to an observer at the Earth's North Pole; currently, this is Polaris. |
| Horizon Coordinate System | A Celestial Coordinate System, uses the observer's local horizon as the fundamental plane, measures the altitude of a celestial body above the horizon and the compass direction. Altitude is the angular distance of a celestial body above the horizon, measured along a vertical circle. Azimuth is angle of the object around the horizon, usually measured from the north point towards the east. |
| Zenith | The highest point directly above the observer, its altitude is 90 degrees; at the horizon its altitude is 0 degrees. |
| Nadir | The direct opposite of Zenith, at given point is the local vertical direction pointing in the direction of the force of gravity at that location. |
| light gathering power | the ability of a telescope to collect as much light as possible so that the faintest object possible can be observed. The its power of an optical telescope is proportional to the diameter^2 (or aperture^2) of the objective lens or mirror. Note that the area of a circle is proportional to the square of the radius. A telescope with a lens which has a diameter three times that of another will have nine times the light-gathering power. Larger objectives gather more light, and more sensitive imaging equipment can produce better images from less light., The ability of a telescope to gather light. Porportional to the area of the telescope objective lens or mirror. |
| aperture | (diameter of the objective lens/mirror) A usually adjustable opening in an optical instrument, such as a camera or telescope, that limits the amount of light passing through a lens or onto a mirror. The diameter of such an opening, often expressed as an f-number. The light-gathering power of an optical telescope is proportional to the diameter^2 (or aperture^2) of the objective lens or mirror. |
| Electromagnetic Spectrum | the distribution of electromagnetic radiation, the range of all possible Electromagnetic Radiation frequency, RADIO, MICROWAVE, INFRARED, VISIBLE LIGHT, ULTRAVIOLET, X-RAY, GAMMA-RAY |
| Speed of Light | 300,000 km/s; 186,000 miles per second |
| ultraviolet, x-ray, gamma-ray | high frequency waves = short wave length = high energy |
| radio, microwave, infrared | low frequency waves = long wave length = low energy |
| Electromagnetic Radiation | (vibrating charged particles) self-propagating waves(disturbance that propagates thru space and time) in vaccum or in matter, with different wavelength(distance between successive crests, troughs, or identical parts of a wave), frequency(number of waves that pass a particular point per unit time), energies |
| Law of Reflection | the angle of incidence equals the angle of reflection |
| Reflection | change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated |
| Refraction | change in direction of a wave due to a change in its speed |
| Chromatic Aberration | as light rays travel through a lens, different wavelength(color) rays are bent by different amount, resulting in different focal points |
| Thin Lens Equation | f=(p*q)/(p+q), 1/f=1/p+1/q, f=focal length p=object distance q=image distance |
| Thin Lens Equation (Astronomical Term) | f=(p*q)/(p+q), 1/f =1/p+1/q, 1/f =1/q, f=q, as object distance is infinite |
| Magnification | M=f(objective)/f(eyepiece), M=f(mirror)/f(lens), The amount of magnification depends on the focal length of the eyepiece LOWER FOCAL LENGTH EYEPIECE MORE MAGNIFICATION POWER |
| focal length | distance between lens & focal point WEAKER EYEPIECE (OCULAR) MORE MAGNIFICATION POWER |
| focal point | point where all the reflected light rays meet |
| optical telescopes | reflective, refractive, catadioptic |
| Objective | main lens or mirror to focus light |
| Eyepiece (ocular) | lens to magnifying image |
| Polarization | Non polarized light vibrates in all planes, polarized light is light that is confined to one plane |
| Reflective Telescope | ADVANTAGE no chromatic aberration, large size capability, visible & shorter wavelength, less expensive inch for inch, portability DISADVANTAGE spherical aberration (Newtonian flat sec. mirror side eyepiece, Cassegranian convex sec. mirror middle eyepiece) |
| spherical aberration | light rays incident on the edges of the spherical mirror are focused at a different point from the light rays incident closer to the center of the mirror, a blurry image results CORRECTED BY USING PARABOLIC MIRROR |
| Refractive Telescope | ADVANTAGE dark background, good resolution, planetary studies, moon, sun DISADVANTAGE more expensive for same aperture, chromatic aberration (Galilean concave eyepiece, Keplerian convex eyepiece) |
| Schmidt Cassegranian telescope | Catadioptic telescope, correcting lens for spherical aberration, middle convex secondary mirror |
| Altazimuth Mounting | a horizontal axe & a vertical axe |
| Equatorial Mounting | one fixed axis parallel with the Earth's rotaional axis to follow diurnal motion |
| Focal Ratio (F-ratio) | focal length(objective) divided by the aperture; Characteristics of a short focal ratio are: Wide field of view, Lower magnification capabilities, Shorter telescope, More suited to Astrophotography - Shorter exposures; Characteristics of a long focal ratio are: Narrow field of view, High magnification capabilities, Less suited to Astrophotography - Longer exposures |
| Resolution/Resolving Power | the ability of a telescope to separate or resolve objects especially binary stars that appear to be so close together as to be a single point to the unaided eye. EX mizar & alcor |
| Binocular | first number denotes the magnification, second number denotes the aperture in mm. |
| Diagonal Prism/Star Diagonal | the eyepiece is fitted into the diagonal to allow more comfortable and convient viewing |
| Focus Knob | to focus the main telescope, closest to the eyepiece |
| R.A. slow motion control knob | vertical to the R.A. setting circle is the knob forfine movements to rotate the telescope along the axis perpendicular to earth's rotational axis |
| Declination slow motion control knob | fine adjustment to the motion parallel to earth's rotational axis |
| Setting Circle | 2 circles with graduated markings that are attached to the axes of the telescope mounting, the circle attached to the base of the mounting is for Right Ascension which is incremented at 5 min intervals, the circle attached to the side of the main scope tube is for Declination and is incremented in degrees. these are used to track sky objects using the Celestial Coordinated System (Equitorial) |
| angular measurement | 1°=60 arc minutes (60')=3600 arc seconds(60") |
| Angular separation | is the angular size of an astronomical object measured in degree, arc minutes or arc seconds |
| Function of telescope | 1:to collect light(most important) 2:to resolve detail 3:to magnify 4:to measure 5:to record |
| most useful magnification | 20 times per cm. of aperture |
| Angular resolution (in arc seconds) | 11.6 / Aperture cm. (theoretical figure) |
| apparent field of view | usually about 45 arc degrees |
| true field of view | apparent field of view (45 arc degrees) / Magnification |
| Altitude | angular distance above the horizon |
| Azimuth | angular measure of compass direction |
| Solstice | the tilt of the Earth's axis is most inclined toward or away from the Sun, causing the Sun's apparent position in the sky to reach its northernmost or southernmost extreme, at most northern June 21(longest day of a year), at most southern Dec 21(shortest day of a year) |
| Sidereal Period | The time it takes for a planet (or other astronomical body) to return to the same angular position (either in its orbit or its rotation) relative to the fixed stars; The period of a planet in its motion around the Sun, or in its rotation on its axis, is the length of time it takes for one revolution or rotation, as viewed by a fixed (non-moving) external observer. |
| Lunar phases | orbit COUNTERCLOCK WISE; New Moon,0°E,6am rises,12pm overhead; Waxing Crescent,45°E,9am rise,3pm overhead; First Quarter,90°E,12pm rise,6pm overhead; Waxing Gibbous,135°E,3pm rise,9pm overhead; Full moon,180°E,6pm rise,12am overhead; Waning Gibbous,135°W,9pm rise,3am overhead; 3rd Quarter,90°W,12am rise,6am overhead; Waxing Crescent,45°W,3am rise,9am overhead |
| Synodic Period | The time a body in the solar system takes to orbit another body once and return to the same orbital relationship. (with respect to Earth in humans perspective) |
| Dinural Motion | The apparent daily motion of celestial bodies across the sky from east to west, caused by the Earth's rotation. The rotation of the Earth on its axis results in the objects in the sky appearing to rise and set each day. |
| Solar Core | not visible, exothermic thermonuclear reaction (nuclear fusion) that convert hydrogen into helium to produces energy |
| Photosphere | visible layer above the solar core, composed of convection cells called granules which is caused by convection currents of plasma within the Sun's convection zone. |
| Chromosphere | immediately above the photosphere, reddish colour characteristic of hydrogen gas, only can be seem during a total solar ecilpse, thin and low in denisty |
| Corona | the layer above Chromosphere, visible during a total solar eclipse, extends millions of miles beyond the photosphere, solar winds expands away as plasmas from the sun at great distance, consists of high velocity charged particles |
| Limb Darkening | non-uniform brightness edges of sun is cooler due to optical depth |
| Sunspots | dark spots on photosphere that are cooler and darker, intense magnetic field, reverse polarity every sunspots cyle of 11 years, umbra is the darkest central region of the sunspot, penumbra is the lighter peripheral region; solar cycle is 22 years reversing back to original polarity |
| Plages | bright regions on chromosphere often appearing prior to a sunspot and surrounding an area above a sunspot; these are regions that are hotter than the surrounding area |
| Faculae | bright granular structures on the Sun's surface that are slightly hotter than the surrounding photosphere, near edges or the limb of the sun, the extensions of it up into the chromosphere become visible over the entire disk in spectroheliograms taken at the wavelengths of hydrogen or ionized calcium vapour. When seen thus away from the limb, they are called chromospheric faculae or plages. |
| Filaments | prominences appears in the disk of the sun, because its cooler temperature, it appears as dark, thread-like structures near the plage region |
| Prominences | flame-like projection extending outward of sunś surface from photosphere outward to corona |
| Spicules | spikes of gas rising from the photosphere, still considered to be permanent features of the chromosphere, near limb appear similar to a forest of trees and last about 20 mins |
| Flares | sudden, rapid, and intense variation in brightness, enormous outburst of magnetic energy, all forms of radiation are ejected into space, affect all layers of the solar atmosphere, photosphere, corona, and chromosphere, most occur in active regions around sunspots, where intense magnetic fields penetrate the photosphere to link the corona to the solar interior |
| Granulation | grainy appearance plasma cells on the sunś disk, convective effect of hot gases rising and cooler gases descending |
| Mare/Maria | large, dark basaltic plains result of ancient volcanic eruption, now covered with a layer of lava dust as much as 5 to 7 miles thick that has been caused by the erosive effects of the solar winds, as well as accumulated meteoric material |
| Lunar Craters | impact craters as a result of meteorite impact, some with sharp mountain peak caused by erupting lava when the impacting meteor melts the moonś crustal material |
| Lunar Scarps | "scraped" by a meteor that hit the surface tangentially, normally found between 2 craters, in mountain ranges or any other extended high surface on the moon; if 2 craters seem to blend together without any wall between them, it is likely that they were scraped by a passing meteoroid |
| Lunar Rilles | long, narrow depressions in the lunar surface that resemble channels, unknown causation, leading theories include lava channels, collapsed lava tubes, near-surface dike intrusion, subsidence of lava-covered basin and crater floors, and tectonic extension |
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