Astronotes
Scopes & Mounts
Telescopes can be broadly classified into 3 categories: Refractors, Reflectors/Newtonians & Catadioptric/Cassegrains. Refractors uses lenses, reflectors use mirrors while cassegrains are modern-day hybrids that combine lenses & mirrors.
In general, Refractors tend to be the most economic option and good for casual observing for brighter objects such as the moon & planets. Reflectors provide the largest bang for the buck, and with larger objectives are more suited for deep sky observation - but tend to be large & bulky. Catadioptrics are more compact and can be excellent scopes but are generally more expensive. Reflectors and Catadioptrics with mirrors require more maintenance as compared to Refractors with lenses.
Mounts may either be Alt-Az or German Equatorial. Alt-Az allow up-down motion (along 90 degrees for altitude above the horizon) and rotational motion along the horizontal plane (along 360 degrees east of north or "azimuth"). Alt-Az mounts are common for starter telescopes and good for casual observing. Two types of Alt-Az mounts include the standard fork-mount (similar to tripods) & Dobsonians, that are commonly combined with Reflectors/Newtonians. However, it is harder to track celestial objects over longer periods of time due to the apparent rotational motion of stars & other objects in the night sky. This problem is overcome by using Equatorial Mounts. Once polar aligned with the celestial pole, the mount needs to move in a circular fashion around only one axis (Right Ascension) to track celestial objects, instead of stepped vertical & lateral adjustments needed for Alt-Az mounts. This makes German Equatorial mounts more suited to the needs of astrophotography.
Night Sky Motion
Stars appear to be affixed to a large, imaginary celestial sphere that moves opposite to the direction of earth's rotational spin, around the polar axis and with a celestial equator, celestial poles & meridian.
The earth's equator projected onto the celestial sphere defines the celestial equator.
At any location, the great circle that passes through the NCP, SCP and the zenith & nadir, is the celestial meridian. The intersection of the celestial equator and the plane of ecliptic (at vernal equinox) determines the 0 deg celestial meridian. This location is also called the First Point of Aries.
The intersection between the celestial meridian and the horizon is North & South.
The intersection between the celestial equator and the horizon is East & West.
The position of stars can be defined by DECLINATION & RIGHT ASCENSION co-ordinates.
DEC specifies angle up or down relative to celestial equator expressed in degrees & minutes (similar to latitude). RA specifies angle east or west relative to (prime) celestial meridian expressed in hours and minutes (similar to longitude). The prime meridian in the sky, designated as 0 hours right ascension, is where the Sun crosses the celestial equator in the northern spring at the Vernal Equinox, a location called the First Point of Aries. Equinox means equal day and night.
In 1 hour the sky appears to turn 15 degrees.
As the earth rotates about its axis west-to-east, the stars appear to spin counter clock-wise east-to-west. In addition, due to the revolution of the earth around the Sun, the stars also have a westward seasonal drift with stars rising 1 degree or 4 minutes earlier than the earlier day. Due to the seasonal drift, different constellations are associated with different times or seasons in the year. For instance, Orion in the winter, Leo in the spring, Scorpio in summer & Pegasus in the fall.
https://en.wikibooks.org/wiki/General_Astronomy
Stars appear to rotate around the polar axis (or around the NCP/SCP) in circles parallel to the celestial equator, rising to the highest altitude at the celestial meridian.
Angle between horizon & pole at any given location is the same as the latitude.
Therefore, the altitude of Polaris at any location is given by its latitude.
At the equator, Polaris is at the horizon (0 deg).
At North Pole, Polaris is at the zenith (90 deg).
Circumpolar stars for any given location are those that do not appear to rise or set and viz. always stay above the horizon. At the equator, there are no visible circumpolar stars, whereas at the poles, all visible stars are circumpolar.
Stars visible in the night sky at any given location (and which ones are circumpolar) depends on its latitude.
Poetry in motion. Each night, every season.
As the earth rotates around the Sun, alternating between day and night, the sky appears to move east to west, with the constellations drifting westward at a speed of 15 degrees every 60 minutes. But since the earth is also revolving around the Sun, the backdrop of the night sky, meaning the part of the neighboring cosmos as viewed from our vantage point from the earth changes every night, with the stars and constellations appearing to rise 1 degree or 4 minutes earlier than the night before. The net result of this phenomenon is that we see different constellations in the night sky across seasons, and different stars reign high in the night skies through the year.
The constellation of Leo shines bright and high in the spring, while Scorpio controls the summer nights. Pegasus takes over in the Fall, while Orion, the great hunter returns every winter. Similar to the constellations, there are different asterisms associated with the seasons around the year. Spica and Arcturus are two prominent stars that make up the Spring Diamond, while Deneb, Vega and Altair form the Summer Triangle. Alpheratz, Algenib, Scheat and Markab build the Great Square of Pegasus, while the winters are graced with the gynormous hexagon built from the stars of Sirius, Procyon, Castor (and Pollux), Capella, Aldebaran and Rigel - notably with the red giant, Betelgeuse not far from its center.
Degrees of Separation
At arm's length
- A handspan is 22 degrees wide
- A fist is 10 degrees wide
- Three fingers is 5 degrees wide
- One finger is 1 degree wide (more than span of the full moon)
Astrobasics
Conjunction is when 2 celestial objects are in line, or close to each other when viewed from the earth, for instance when a planet is in line with the Sun.
When a smaller celestial body moves across a larger one, that is termed as Transit. For instance, when an inferior planet is in conjunction with the Sun, and travels in front & across, a transit has occurred.
When a celestial body moves across another such that it briefly blocks the view of the other, an occultation has occurred.
An eclipse is an instance of occultation.
A conjunction with an outer planet is also called opposition.
Inner planets are best viewed at greatest elongation while outer planets are best viewed at opposition.
Astronomical Unit (AU): Mean earth to sun distance. Roughly 150 million km.
Light Year (LY): Distance traveled in 1 year by light moving at roughly 300,000 km/s
Sidereal day is time taken by the earth to rotate / spin exactly 360 degrees around its axis.
When the earth has rotated exactly 360 degrees, the position of the Sun in the sky is not identical to the last instance.
Solar day is time elapsed between noon-to-noon, ie for the Sun to show up in identical positions in the sky relative to a given location on Earth. When the Sun is in the identical position in the sky as the last instance, the Earth has rotated beyond 360 degrees.
Sidereal day is 23hrs 56 min, while a solar day is 24 hours. Sidereal day is shorter than a solar day due to the fact that the earth revolves in its orbit as it rotates about its axis.
Absolute magnitude of a star is related to its luminosity, while the apparent magnitude also factors in distance. Apparent magnitude is more common.
Stars can be classified by the spectral type into O,B,A,F,G,K,M categories - with O being blue/hottest upto 50,000K and M being red/coolest as low as 2000K. The Sun is type G with surface temp of ~5000K. Each category can be further subdivided into numerical classes 0-9, 0 being hotter than 9.
For stars in main sequence, the luminosity & hence absolute magnitude increases at the hotter end of the spectral scale (O v/s M). Therefore for most stars, blue stars are brighter than the red ones.
Some of the giants/supergiants that are close to the end of their life are M spectral type stars - yet have high luminosity & absolute magnitude. Dwarf-stars on the other hand, have low luminosity irrespective of their spectral class (white dwarfs or dying stars v/s brown dwarfs or failed stars).
Plotting the luminosity/absolute magnitude classification v/s the spectral classification/surface temperature gives the Hertzsprung Russell diagram.
http://universe-review.ca/I08-01-HRdiagram1.jpg
Most stars have near uniform luminosity, but some show periodic fluctuations in brightness, and are termed as variable stars. One of the most common examples of intrinsic variables include Cephid variables whose luminosity is related to the periodicity of variation. This helps to calculate absolute magnitude, and knowing the apparent magnitude, the distance of the star can be accurately calculated. This makes Cephid variables a standard candle, similar to a Type1a supernova. Extrinsic variables have near constant luminosity but periodic variations in their apparent magnitude / as observed from the Earth due to rotation or eclipsing binaries or Algol variables.
Binary star systems are common, but sometimes they may also be optical doubles where the stars appear to be close to one another to the observer on Earth, but in reality are spaced apart by great distances.
A standard candle is a class of astrophysical objects, such as supernovae or variable stars, which have known luminosity or absolute magnitude. Since apparent magnitude is also known, this helps accurately predict distance to that object.
Asteroids are rocky bodies that are found between the orbits of Mars & Jupiter. Comets are composed of frozen ice, gas & stellar dust that revolve around the sun in highly elliptical orbits. Shorter orbit comets (<200 years) typically originate in the trans-Neptunian Kuiper belt (that is also home to the dwarf planets of Pluto, Ceres, Eris), but longer range comets emerge from the Oort Cloud located in the most distant realms of the Solar System. Meteoroids are smaller particles of space debris / stellar dust - remnants from the creation of the solar system, floating around the Plane of the Ecliptic. As the earth revolves in its orbit, when meteroids enter the earth's atmosphere, they burn up resulting in meteors (or meteor showers or shooting stars). Occasionally, they may not burn completely and crash into the earth as meteorites, forming meteor-craters.
Time taken by the moon to complete an orbit around the earth is a sidereal month (27+ days).
Time taken by the moon to go through its phases (new, crescent, quarter, gibbous, full) is a synodic month (29+ days). The ,moon has to travel more than 360 degrees to return to the same position over the earth.
Eclipses occur when the moon is in new or full phase & when the moon's orbit intersects the orbit of the earth (nodes). Umbra is region of total eclipse while penumbra is partial.
Ecliptic is the plane along which the planets orbit around the Sun. The ecliptic as viewed from the earth, is an imaginary band in the sky the Sun moves around in. There are 12 constellations in this ecliptic (that define the Zodiac).
The earth's axis of rotation is inclined at 23.5deg to the orbit of revolution or the plane of ecliptic.
Due to the angle of inclination of the earth's spin, as the earth revolves around the Sun, the overhead Sun appears to move north & south between the Tropics.Anywhere between the Tropics, the Sun will pass directly overhead at least twice during the course of a full year. The Sun will never be directly overhead for any location beyond the Tropics.
At vernal equinox (late March), the Sun is directly overhead at the equator, traveling north. 12 hour day & 12 hour night. Beyond vernal equinox, days get longer than nights in the Northern Hemisphere and shorter in the Southern. At solstice (June 21), the Sun is at its northern most location & directly overhead the Tropic of Cancer. Longest day in the northern H, shortest day in the southern H.
Beyond June 21, the Sun now begins its southbound journey towards the equator. At autumn equinox, the Sun is again directly overhead the equator, again generating equal durations for day & night. After autumn equinox, days get shorter than the nights in the Northern Hemisphere and longer in the Southern. At solstice (Dec 22), the Sun is at its southernmost location & directly overhead the Tropic of Capricorn. Shortest day in the northern H, longest day in the southern H.
On Jun 21st, the Sun can be viewed in northern skies - at its highest point from the Northern H, and at its lowest point from the Southern H. Likewise, on Dec 22nd, the Sun can be viewed in southern skies - at its lowest point from the Northern H, and at its highest point from the Southern H.
Position of the Sun along the ecliptic in the celestial sphere:
Vernal equinox / Mar 20: 0 deg DEC. 0hrs RA
Summer solstice/ Jun 21: 23.5 deg N DEC, 6hrs RA
Autumn equinox/ Sep 23: 0 deg DEC, 12 hrs RA
Winter solstice/ Dec 22: 23.5 deg S DEC, 18hrs RA
As protostars evolve, when the accretion disk is not large enough to build enough mass to sustain pressure to generate fusion, these generate failed stars called as brown dwarfs. If the mass is barely sufficient to trigger nuclear fusion, these result in small red dwarfs that have extremely long lives. When the nuclear fuel ultimately run out, red dwarfs finally turn into cold black dwarfs.
Larger stars (comparable to the sun) spend most of their lifetimes in Main Sequence, with expanding forces from fusion balancing the inward compression from gravity. Nearer to the end of their lifetimes, these first turn into red giants, and finally after the outer layers are lost as planetary nebula, the inner cores finally settle as dense white dwarfs, eventually cooling to black dwarfs.
Mass of the stars determines their final fate. For most starts in main sequence, if the final core mass is 1.44 times solar mass or less (Chandrasekhar limit), these will evolve into white dwarfs. However, for larger red giants & blue / white hypergiants, if the core mass exceeds Chandrasekhar limit, these will generate highly compact remnants called as neutron stars. Pulsars & Magnetars are examples of neutron stars. When core mass exceeds 2-3 times solar mass, this results in formation of stellar Black Holes.
Stars that tend to conglomerate together in groups not large enough to be galaxies in itself are called as clusters. Open Clusters tend to have fewer & younger stars. Pleiades is an example of tan open cluster. Globular Clusters have larger numbers of stars (may be in the millions) - and the stars are older (and sometimes nearly as old as the universe itself).
Parallax is the phenomenon when the position of foreground objects seem to shift when the perspective (or location) of an observer changes. For instance, the moon might appear to rise in front of different stars in the background, depending upon location of the observer on earth. Parallax is commonly used to measure distance of nearby stars as the position of stars seem to change slightly with the changing position of the earth revolving in its orbit.
Professional astronomers often use another unit for big distances: the parsec. One parsec equals 3.26 light-years. (In case you're really wondering, a parsec is the distance where a star shows a parallax of one arcsecond against the background sky when the Earth moves 1 a.u. around the Sun.)
Observing the Moon
As the moon progresses into its synodic cycle, moonrise gradually moves from sunrise through the day into the night, while moonset progresses from the sunset through the night into the day.
The phase between Waxing Crescent through the Full Moon is generally best for observing the moon, since the moon tends to be higher in the sky (when viewing conditions are typically best and less interrupted by atmospheric disturbances) during more social hours of the evening & the night.
The phases between Waning Crescent through the New Moon is not optimal for moon observations, since the moon tends to rise higher in the sky late through the night or in the early hours of the morning; however this means the social hours of the evening / night are free of getting washed out by bright moonlight, making those times more suited to general astronomy.
Crescent at sunrise / east in the morning = Waning Phase (Left side of moon is lit)
Crescent at sunset / west in the evening = Waxing Phase (Right side of moon is lit)
Path of the Sun & Moon across the Sky through the Seasons
At the end of Winter Solstice, the sun begins its northbound journey through the Spring Equinox to the Summer Solstice. After Summer Solstice, the sun once again resumes southbound travel through the Autumn Equinox upto the Winter Solstice.
At the Spring Equinox, the sun rises exactly in the east and sets in the west.
After the Spring Equinox through the Summer Solstice, and upto the Autumn Equinox, the sun rises North of the East and sets North of West. The day of the Summer Solstice marks an extremity of the northbound drift of the sun, following which the sun begins its southbound journey.
At the Autumn Equinox, the sun once again rises exactly in the east and sets in the west.
After the Autumn Equinox, through the Winter Solstice, and upto the Spring Equinox, the sun now rises South of East and sets South of West. The day of the Winter Solstice marks the extremity of the southbound drift of the sun, following which the sun now begins it northbound travel.
In the winter months, from autumn to spring equinoxes, when the Sun is rising South of East, the full moon rises North of East while the new moon rises South of East. Waxing moons are longer up in the night skies than waning moons.
In the summer months from spring to autumn equinoxes, when the Sun is rising North of East, the full moon rises South of East while the new moon rises North of East. Waning moons are longer up in the night skies than waxing moons.
The full moon opposes the Sun. The new moon follows the sun.
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