PHYC 2451 Quiz: Formula_quiz1
DepartmentPhysics & Atmospheric Science
Course CodePHYC 2451
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Angular size/diameter (angle subtended by the diameter of an object, apparent size of an object as seen from a given position, “visual diameter” of the
object measured as an angle): 1°=60’(arcmins)=3600”(arcsecs) moon= 1/2°=30’,Venus=30”;
Small-angle formula D= αd/206,265 (D linear size of object, d distance, α angular size of object in arcsec)
At a distance of 1 parsec, a length of 1 AU subtends an angle of 1 arcsec.
1 parsec (pc) =3.26 light-years (ly); 1 ly=9.5E15m=6.3E4AU; 1 astronomical unit (AU) =1.5E11m
Parsec ≡ the distance at which 1 AU perpendicular to the observer’s line of sight subtends an angle of
1 arcsec. (a unit of distance; corresponding to a parallax of one arcsecond; the distance from the Sun to an
astronomical object which has a parallax angle of one arcsecond)
Parallax: the apparent displacement of an object due to the motion of the observer; displacement or difference in the apparent position of an object
viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.
Nebula: a cloud of interstellar gas and dust. Pulsar: a pulsating radio source thought to be associated with a rapidly rotating neutron star.
Supernova: a stellar (relating to stars) outburst during which a star suddenly increases its brightness roughly a million-fold.
Quasar: a very luminous (emitting light, shining, bright) object with a very large redshift and a starlike appearance. (very energetic and distant active
galactic nucleus; among the most distant objects in the universe and outshine normal galaxies by a factor of a hundred)
Constellation: a configuration of stars in the same region of the sky (an internationally defined area of the celestial sphere; a grouping of stars on the
celestial sphere perceived as a figure or design)
Diurnal motion: any apparent motion in the sky that repeats on a daily basis, such as the rising and setting of stars; it’s a consequence
of Earth’s rotation. (Relating to or occurring in a 24-hour period; motion that occurs during the day or daily)
Celestial sphere: an imaginary sphere of very large radius centered on an observer; the apparent sphere of the sky (an imaginary
sphere of infinitely large radius enclosing the universe so that all celestial bodies appear to be projected onto its surface)
Celestial equator: The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it
is a projection of the terrestrial equator out into space. As a result of the Earth's axial tilt, the celestial equator is inclined by 23.4° with respect to the
North celestial pole: point directly above Earth’s North Pole where Earth’s axis of rotation, if extended, would intersect the celestial
Zenith: the point in the sky directly overhead an observer anywhere on Earth.
Circumpolar: stars that neither rise nor set but appears to rotate around one of the celestial poles. (Denoting a star that from a given observer's
latitude never sets below the horizon or never rise above the horizon)
Declination: analogous to latitude, the declination of an object is its angular distance north or south of the celestial equator,
measured along a circle passing through both celestial poles. The declination of the Sun on December 21st will be -23.5°E
Vernal equinox: the point on the ecliptic where the sun crosses the celestial equator from south to north. Also used to refer to
the date on which the Sun passes through this intersection on about March 21.
Right ascension: the angular distance from the vernal equinox eastward along the celestial equator to the circle used in
measuring its declination. It is measured from a line that runs on the celestial equator and passes through the point vernal equinox.
Altitude or elevation of an object in the sky is its angle above the horizon; azimuth or bearing is its angle measured from the
North point of the horizon eastward around to the point on the horizon directly below the objest.
EX: 1. The star lies exactly halfway vernal equinox and south celestial pole: R.A.=0h0m0s, Decl.=-45°0’0” 2. At midnight local time u see a star
with R.A.=2h30m0s at zenith. Time to see a star at zenith with R.A.=21h0m0s. (The time required for the sky to rotate through the angle
between the stars is the difference in their ascensions: 21h0m0s-2h30m0s=18h30m0s. so the 2nd star will be at zenith 18.5h after the 1st one, that
is 6:30pm the following evening ) 3. Star A setting in the west, altitude=0°, azimuth=270°; Star B at North celestial pole as seen @Halifax, alt=45°,
az=0°; Star C at the intersection of the Celestial Equator and Meridian @Halifax, alt=45°, az=180°; Star D at zenith, alt=90°, no azimuth. 4. Living @
altitude 40°N, at the vernal equinox, the Sun is above southern horizon at noon with alt=90°-40°=50°; at the winter solstice, (23.5° lower than the
celestial equator), alt=50°-23.5°=26.5° 5. A star on the meridian at midnight on Sept 23rd, right ascension =0h0m0s; when the vernal equinox rise,
the sidereal time =18h (R.A.=0h0m0s, its sidereal time when crosses the upper meridian at midnight is 0:00, Sun rising from horizon to zenith is 6h,
so 24h-6h=18h); On autumnal equinox, the sidereal time is closely equal to the solar time(Earth need to rotate a complete circle to face the Sun).
Ecliptic plane: the plane of Earth’s orbit around the Sun. Ecliptic: the circular path that the Sun appears around the celestial sphere
Autumnal equinox: the intersection of ecliptic and the celestial equator where the Sun crosses the equator form north to south, also refer to the
date. On about Sept 22, the sun moves southward across the celestial equator at this point. Summer solstice: the point on the ecliptic farthest
north of the celestial equator.
Zodiac: a band of 12 constellations around the sky centered on the ecliptic; the moon’s path also remains within the band.
Procession: a slow, conical motion of Earth’s axis of rotation caused by gravitational pull of the Moon and Sun on Earth’s equatorial
bulge. (The Earth's axis rotates (precesses) just as a spinning top does. Period:about 26,000 years. The North Celestial Pole will not always be point
towards the same starfield. Caused by the gravitational pull of the Sun and the Moon on the Earth.) < The change in tilt of Earth's axis with respect
to the Stars after six months has elapsed 0E >
Procession of the equinoxes: the slow westward motion of the equinoxes along the ecliptic due to procession of Earth
Meridian: the great circle on the celestial sphere that passes through an observer’s zenith and the north and
south celestial poles. (the great circle on the celestial sphere passing through the north and south celestial
poles and the zenith and nadir of the observer)
Meridian transit: the crossing of the meridian by any astronomical object. If the crossing occurs above the
horizon,--upper meridian transit. Not necessarily at the zenith directly overhead, but is at the highest point above
the horizon that you will see it from your location.
Apparent solar day: interval btw two successive upper meridian transits of the Sun as observed from any fixed point on Earth.
Sidereal period: the orbital period of one object about another as measured with respect to the stars. The true orbit period of
a planet to complete one full orbit of the Sun relative to the stars.
Sidereal year: orbital period of Earth about Sun w.r.t the stars. Sidereal month: period of Moon’s revolution
about Earth w.r.t. the stars. Sidereal day: the time between two successive upper meridian passages of the vernal
equinox; the interval between successive meridian passages of the vernal equinox. 1 sidereal day = 23h56m4.091s <The stars
rise 4 minutes earlier each day. A star 8:30 on Nov 1st, 8:02 on Nov 8th(8:30-4*( 8th - 1st )>
Sidereal month: time it takes the Moon to complete one full orbit of Earth, as measure w.r.t the starts.
Synodic month, lunar month: time it takes the Moon
to complete one cycle of phases, measured w.r.t. the
Sun. from new moon to new moon. 29.53 days.
The First Point of Aries is the location of the vernal equinox, and is named for the constellation of Aries. It is one
of the two points on the celestial sphere at which the celestial equator meets the ecliptic plane, the other being the
First Point of Libra, located exactly 180° from it. The First Point of Aries is so called because, when Hipparchus defined
it in 130 BCE, it was located in the western extreme of the constellation of Aries. Due to Earth's axial precession, this
point gradually moves westwards at a rate of about one degree every 72 years. Since the time of Hipparchus, it has
shifted across the sky by about 30°, and is currently located within Pisces.
If you see a First Quarter Moon at sunset in late September, which way are you looking? d)Winter Solstice
Retrograde motion is motion in the direction opposite to the movement of something else, and is the contrary of
direct or prograde motion. In the Solar System, most planets rotate in the same prograde direction, the exceptions
being Venus and Uranus, which have retrograde rotations.
Lunar phases: the different appearances of the Moon. The Moon rises and sets about an hour later
each night. New moon: the Moon is barely visible for a day. Only located near the Sun in the sky, it rises around
sunrise, sets around sunset. First quarter moon: half is illuminated, ¼ of the way around the celestial
sphere from the Sun. It rises and sets about ¼ of an Earth rotation: Moonrise occurs around noon, moon set occurs
around midnight, highest in the sky is around 6pm. Full moon: Moon stands opposite the Sun in the sky; it rises at
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