comets
Actually, since comets are made of ice, and dust and rock, it takes quite a bit of luck for it to "light up" when it comes near the sun. Only once in a while you get comets that are spectacular and brighten up sufficiently to be seen with the naked eye. The rest just remain as dustballs that are binocular or telescopic only. Anyway, that picture u put up is probably a composite, the rock was added later...
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zong
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Haha thanks quasar!! I only realised the rock is artificial after you pointed it out! I was looking at the comet only, not noticing anything else..
Actually, from the laws of physics, it is impossible for the comet to go totally the reverse direction as the tail. The comet can only move about 90degress to the tail of the sun, so in the picture above can only move top-left or bottom-right, at maximum, not top-right. However the details abit too confusing, so I shan't explain it here.. It's got something to do with how the comet moves in its orbit. Oh dear I've contradicted Richard twice already hope i don't anger you hehe..
Also, it's not correct to say all the tails of the comet point away from the sun. As you can see from the picture above, the "top" tail is actually the one pointing away from the sun, I know because it is VERY straight. For the "bottom" tail, you can see it's a small curve, which is actually it's orbit around the sun. That's the "dust tail" of the comet, while the one directly opposite the sun I forgot its name. Some comets even have 3 tails! But, there can only be 1 tail opposite to the sun.
Actually, from the laws of physics, it is impossible for the comet to go totally the reverse direction as the tail. The comet can only move about 90degress to the tail of the sun, so in the picture above can only move top-left or bottom-right, at maximum, not top-right. However the details abit too confusing, so I shan't explain it here.. It's got something to do with how the comet moves in its orbit. Oh dear I've contradicted Richard twice already hope i don't anger you hehe..
Also, it's not correct to say all the tails of the comet point away from the sun. As you can see from the picture above, the "top" tail is actually the one pointing away from the sun, I know because it is VERY straight. For the "bottom" tail, you can see it's a small curve, which is actually it's orbit around the sun. That's the "dust tail" of the comet, while the one directly opposite the sun I forgot its name. Some comets even have 3 tails! But, there can only be 1 tail opposite to the sun.
anti-tail or anomalous tail
When a comet's tail appears to be pointing toward the Sun, this is called an anti-tail or anomalous tail. In reality, the tail only appears to be pointing toward the Sun. To get an anti-tail, the comet must produce large ("heavy") dust particles. If this happens, these particles are left along the comet's orbit instead of being pushed away from the Sun and the comet's orbit by light pressure. Often dusty comets will produce particules of different sizes creating a fan-shaped appearance. The smallest dust will be pushed directly away from the Sun (like the gas tail) and the largest will be left in the comet's orbit. When a comet is close to the Sun, the angle of this fan can be 90 degrees or larger. If the Earth-comet-Sun geometry is correct, the dust in the comet's orbit will appear to point toward the Sun. [Try this...make a right (90 degree) angle with your thumb and index finger. Your index finger is the main tail and your thumb is the dust left in the comet's orbit. Point your finger and thumb directly away from you (keeping the angle 90 degrees). Your finger seems to be going in exactly the opposite direct from the thumb. This is what causes an anti-tail.
Courtesy of Charles S. Morris, JPL
Rest of the site is here: http://encke.jpl.nasa.gov/define.html
Lots of info there...
Basically, I think it means that it appears to oppose the sun coz it's the material that's left behind when a comet is moving away from the sun.
When a comet's tail appears to be pointing toward the Sun, this is called an anti-tail or anomalous tail. In reality, the tail only appears to be pointing toward the Sun. To get an anti-tail, the comet must produce large ("heavy") dust particles. If this happens, these particles are left along the comet's orbit instead of being pushed away from the Sun and the comet's orbit by light pressure. Often dusty comets will produce particules of different sizes creating a fan-shaped appearance. The smallest dust will be pushed directly away from the Sun (like the gas tail) and the largest will be left in the comet's orbit. When a comet is close to the Sun, the angle of this fan can be 90 degrees or larger. If the Earth-comet-Sun geometry is correct, the dust in the comet's orbit will appear to point toward the Sun. [Try this...make a right (90 degree) angle with your thumb and index finger. Your index finger is the main tail and your thumb is the dust left in the comet's orbit. Point your finger and thumb directly away from you (keeping the angle 90 degrees). Your finger seems to be going in exactly the opposite direct from the thumb. This is what causes an anti-tail.
Courtesy of Charles S. Morris, JPL
Rest of the site is here: http://encke.jpl.nasa.gov/define.html
Lots of info there...
Basically, I think it means that it appears to oppose the sun coz it's the material that's left behind when a comet is moving away from the sun.
Hi Zong, Quasar,
The rock actually is real, but was illuminated briefly with a flashlight in the long exposure shot. The explanation is found here:
http://antwrp.gsfc.nasa.gov/apod/ap040420.html
Cheers
Chris
The rock actually is real, but was illuminated briefly with a flashlight in the long exposure shot. The explanation is found here:
http://antwrp.gsfc.nasa.gov/apod/ap040420.html
Cheers
Chris
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zong
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Huh?! Okie.. I realised that I haven't explained it myself after posting quite a number of times here lemme try explaining...
Suppose a comet is staying still right at its position, not moving at all. Of course in reality this can't be so, but let's first assume that, ok? Good So now we know that comets are made of dust, (abit of) rock, and mostly ice. And we know that the sun, while burning its hydrogen, gives off small particles other than photons(aka. light), and these small particles collectively are known as the "solar wind".
These small particles must travel somewhere right? So they travel away from the sun. Eventually, some of these particles must hit the comet right? and in which direction do they hit it? Of course, from the sun toward the comet, and going away from the sun.
When these small particles hit the comet, it brings with it some ice or dust particles from the comet itself. As they travel at VERY high velocities, hitting on some ice or dust particles will not have an impact on its path and it continues forward, that is, against the direction of the sun, thus creating the anti-tail. Therefore, you can see that the anti-tail always points away from the sun. The nearer the comet to the sun, the more solar wind on it, and thus a longer anti-tail.
The other tails that I mentioned are a result of the comet's orbit around the sun. Imagine you're the comet, holding a sack of rice (imagine the rice is the comet's ice), and the sack has a small opening, which leaks rice as you move. Over time, you would thus see that the comet sheds some particles in its path, which accounts for its dust tail. So, the dust tail itself is already a sign that the comet is in motion. It moves at a great speed, but still it's a speed that we cannot measure with our naked eyes, and so the motion cannot be detected.
Hope this helps!
lemme try explaining... Suppose a comet is staying still right at its position, not moving at all. Of course in reality this can't be so, but let's first assume that, ok? Good
So now we know that comets are made of dust, (abit of) rock, and mostly ice. And we know that the sun, while burning its hydrogen, gives off small particles other than photons(aka. light), and these small particles collectively are known as the "solar wind". These small particles must travel somewhere right? So they travel away from the sun. Eventually, some of these particles must hit the comet right? and in which direction do they hit it? Of course, from the sun toward the comet, and going away from the sun.
When these small particles hit the comet, it brings with it some ice or dust particles from the comet itself. As they travel at VERY high velocities, hitting on some ice or dust particles will not have an impact on its path and it continues forward, that is, against the direction of the sun, thus creating the anti-tail. Therefore, you can see that the anti-tail always points away from the sun. The nearer the comet to the sun, the more solar wind on it, and thus a longer anti-tail.
The other tails that I mentioned are a result of the comet's orbit around the sun. Imagine you're the comet, holding a sack of rice (imagine the rice is the comet's ice), and the sack has a small opening, which leaks rice as you move. Over time, you would thus see that the comet sheds some particles in its path, which accounts for its dust tail. So, the dust tail itself is already a sign that the comet is in motion. It moves at a great speed, but still it's a speed that we cannot measure with our naked eyes, and so the motion cannot be detected.
Hope this helps!
Hi fiona,
this are some terminology on comets that might be useful to u.
absolute magnitude (Ho)
The brightness of a comet when it is at 1 AU from both the Earth and Sun. As this virtually never happens, this quantity is calculated from the comet's light curve. Unfortunately, this quantity is far from absolute. It can be different pre- and post-perihelion. It can also change from apparition to apparition (for periodic comets).
anti-tail or anomalous tail
When a comet's tail appears to be pointing toward the Sun, this is called an anti-tail or anomalous tail. In reality, the tail only appears to be pointing toward the Sun. To get an anti-tail, the comet must produce large ("heavy") dust particles. If this happens, these particles are left along the comet's orbit instead of being pushed away from the Sun and the comet's orbit by light pressure. Often dusty comets will produce particules of different sizes creating a fan-shaped appearance. The smallest dust will be pushed directly away from the Sun (like the gas tail) and the largest will be left in the comet's orbit. When a comet is close to the Sun, the angle of this fan can be 90 degrees or larger. If the Earth-comet-Sun geometry is correct, the dust in the comet's orbit will appear to point toward the Sun. [Try this...make a right (90 degree) angle with your thumb and index finger. Your index finger is the main tail and your thumb is the dust left in the comet's orbit. Point your finger and thumb directly away from you (keeping the angle 90 degrees). Your finger seems to be going in exactly the opposite direct from the thumb. This is what causes an anti-tail.]
apparition
The time during which a comet is under observation. For periodic comets which have more than one appearance, the term apparition is often used with the year of perihelion passage, such "the 1910 apparition of Comet Halley." The term probably is derived from the ghostly appearance of bright naked-eye comets.
astronomical unit (AU)
Standard unit for measuring distance within the solar system. One AU is equal to the average distance between the Sun and Earth or about 93 million miles.
coma or the comet's head
The comet's coma or head is the fuzzy haze that surrounds the comet's true nucleus. The coma (and tail) are really all that we see from Earth.
The shape of the coma can vary from comet to comet and for the same comet during its apparition. The shape depends on the comet's distance from the Sun and the relative amount of dust and gas production. For faint comets or bright comets producing little dust, the coma is usually round. Comets, which are producing significant quantities of dust, have a fan-shaped or parabolic comae. This is because different size dust is being released. The larger dust gets left along the comet's orbital path while smaller dust gets pushed away from the Sun by light pressure. The smaller the dust, the more directly away from the Sun the dust is pushed. With a distribution of both large and small dust sizes, a fan is created. For comets within 1 AU, the coma of a dusty comet often becomes parabolic in shape. Clearly, for comets with fan-shaped or parabolic comae, there is no obvious boundary between the coma and tail.
coma diameter
The diameter of the coma is usually given in minutes of arc ('). If the coma is round, this is a straightforward definition. If the coma is elongated or has a tail, the measurement represents the smallest dimension of the coma (usually at a right angle to the tail) and transecting the brightest part of the coma.
degree of condensation (DC)
DC is an indicator of how much the surface brightness of the coma increases toward the center of the coma. In general, DC=0 indicates totally diffuse and DC=9 means "stellar." As the DC increases, the coma size usually decreses and becomes more sharply defined. A totally diffuse comet, with no brightening toward the center, is rated DC=0. With DC=3-5, there is a distinct brightening. By DC=7 you have a steep overall gradient and by DC=8 the coma is very small, dense,and intense with fairly well defined boundaries. With DC=9 the comet looks like a soft star or a planet in bad seeing.
geocentric distance (delta)
The comet's distance from the Earth in astronomical units.
heliocentric distance (r)
The comet's distance from the Sun in astronomical units.
long-period comets
Comets with orbital periods greater than 200 years.
"n"
The photometric parameter n in the power-law formula for comet brightness, m1 = Ho + 5 log (delta) + 2.5n log (r), indicates how fast the comet's brightness is changing with heliocentric distance, r. Specifically, n is the power in the power-law formula. That is, the comet's brightness varies as r to the -n power. When the comet's heliocentric brightness, m1 - 5 log (delta), is plotted against log (r), the slope of the straight line (assuming it is a straight line) is 2.5n.
nucleus
The true nucleus of a comet has only been seen once (P/Halley by spacecraft). From the ground, the star-like nucleus always includes a cloud of dust and gas around the true nucleus. Hence, terms such as stellar condensation and nuclear condensation are often used when a star-like object is seen in the comet's coma. The magnitude of the "nucleus" is denoted m2 and usually isn't of much use because one is really not such what m2 represents. In general, the value of m2 will get fainter as more magnification is applied.
observed magnitude (m1)
The observed magnitude of the comet represents the integrated brightness of the comet's coma or head as seen from Earth. This is normally obtained by comparing the comet's average surface brightness with that of defocused stars (matching the comet's size) of known brightness. Because comets have size (in contrast to stars which are pinpoints of light), a comet of a given brightness will appear less obvious than a star of the same brightness.
opposition
The comet is in the midnight sky on the opposite side of the Earth from the Sun. A perfect opposition, which almost never happens, has the comet 180 degrees away from the Sun.
outburst (in brightness)
An unexpected increase in brightness over a short period of time due to the release of dust and gas into the coma from the nucleus. For a visual observer, the nuclear condensation (a bright spot near the center of the coma) will appear to become star-like and brighter in the comet's coma. Over time (hours - days), the size of the nuclear condensation will increase as the dust moves away from the nucleus. The change in brightness can be as little as half a magnitude and as much as many magnitudes.
perihelion
The point in a comet's orbit that it is closest to the Sun.
perihelion date
The date (and time) the comet reaches perihelion.
perihelion distance
The comet's distance from the Sun, usually expressed in Astronomical Units, at perihelion.
periodic or short-period comets
Any comet with an orbital period of less than 200 years. These comets are indicated by a "P/" before the names. For example, P/Halley is Halley's comet or more properly known as periodic Comet Halley. Recently, the International Astronomical Union has started numbering periodic comets that have been seen at more than one apparition. Thus, Halley's Comet is 1P/Halley and P/de Vico is now known as 122P/de Vico.
position angle (PA)
The PA of a tail or other cometary feature represents the direction on the sky (in degrees from north) toward which it is pointing. Thus, a comet in the morning sky (in the east) that has a tail pointing due west will have a PA of 270 degrees. A comet in with a tail poining toward the south-east will have a PA of 135 degrees. It must be stressed that the determination of the PA of a tail (or other feature) requires plotting it on an atlas and measuring the angle with a protractor. PAs should be measured to at least five degree resolution. It is not possible to look in an eyepiece and accurately estimate the PA of a tail. Also, the determination of PA in the polar regions of the sky is very tricky and may not be intuitive.
solar conjunction
The comet is near the Sun in the sky. Usually, this means that the comet will not be observable from Earth.
tail
The comet's tail is its most distinctive feature. Generally pointing away from the Sun, these appendages come in a variety of shapes and lengths. The lengths can vary from a small fraction of a degree (tails are always measured as the angular length either in degrees or minutes of arc [', 60' = one degree]) to the rare few that cover a significant fraction of the sky.
this are some terminology on comets that might be useful to u.
absolute magnitude (Ho)
The brightness of a comet when it is at 1 AU from both the Earth and Sun. As this virtually never happens, this quantity is calculated from the comet's light curve. Unfortunately, this quantity is far from absolute. It can be different pre- and post-perihelion. It can also change from apparition to apparition (for periodic comets).
anti-tail or anomalous tail
When a comet's tail appears to be pointing toward the Sun, this is called an anti-tail or anomalous tail. In reality, the tail only appears to be pointing toward the Sun. To get an anti-tail, the comet must produce large ("heavy") dust particles. If this happens, these particles are left along the comet's orbit instead of being pushed away from the Sun and the comet's orbit by light pressure. Often dusty comets will produce particules of different sizes creating a fan-shaped appearance. The smallest dust will be pushed directly away from the Sun (like the gas tail) and the largest will be left in the comet's orbit. When a comet is close to the Sun, the angle of this fan can be 90 degrees or larger. If the Earth-comet-Sun geometry is correct, the dust in the comet's orbit will appear to point toward the Sun. [Try this...make a right (90 degree) angle with your thumb and index finger. Your index finger is the main tail and your thumb is the dust left in the comet's orbit. Point your finger and thumb directly away from you (keeping the angle 90 degrees). Your finger seems to be going in exactly the opposite direct from the thumb. This is what causes an anti-tail.]
apparition
The time during which a comet is under observation. For periodic comets which have more than one appearance, the term apparition is often used with the year of perihelion passage, such "the 1910 apparition of Comet Halley." The term probably is derived from the ghostly appearance of bright naked-eye comets.
astronomical unit (AU)
Standard unit for measuring distance within the solar system. One AU is equal to the average distance between the Sun and Earth or about 93 million miles.
coma or the comet's head
The comet's coma or head is the fuzzy haze that surrounds the comet's true nucleus. The coma (and tail) are really all that we see from Earth.
The shape of the coma can vary from comet to comet and for the same comet during its apparition. The shape depends on the comet's distance from the Sun and the relative amount of dust and gas production. For faint comets or bright comets producing little dust, the coma is usually round. Comets, which are producing significant quantities of dust, have a fan-shaped or parabolic comae. This is because different size dust is being released. The larger dust gets left along the comet's orbital path while smaller dust gets pushed away from the Sun by light pressure. The smaller the dust, the more directly away from the Sun the dust is pushed. With a distribution of both large and small dust sizes, a fan is created. For comets within 1 AU, the coma of a dusty comet often becomes parabolic in shape. Clearly, for comets with fan-shaped or parabolic comae, there is no obvious boundary between the coma and tail.
coma diameter
The diameter of the coma is usually given in minutes of arc ('). If the coma is round, this is a straightforward definition. If the coma is elongated or has a tail, the measurement represents the smallest dimension of the coma (usually at a right angle to the tail) and transecting the brightest part of the coma.
degree of condensation (DC)
DC is an indicator of how much the surface brightness of the coma increases toward the center of the coma. In general, DC=0 indicates totally diffuse and DC=9 means "stellar." As the DC increases, the coma size usually decreses and becomes more sharply defined. A totally diffuse comet, with no brightening toward the center, is rated DC=0. With DC=3-5, there is a distinct brightening. By DC=7 you have a steep overall gradient and by DC=8 the coma is very small, dense,and intense with fairly well defined boundaries. With DC=9 the comet looks like a soft star or a planet in bad seeing.
geocentric distance (delta)
The comet's distance from the Earth in astronomical units.
heliocentric distance (r)
The comet's distance from the Sun in astronomical units.
long-period comets
Comets with orbital periods greater than 200 years.
"n"
The photometric parameter n in the power-law formula for comet brightness, m1 = Ho + 5 log (delta) + 2.5n log (r), indicates how fast the comet's brightness is changing with heliocentric distance, r. Specifically, n is the power in the power-law formula. That is, the comet's brightness varies as r to the -n power. When the comet's heliocentric brightness, m1 - 5 log (delta), is plotted against log (r), the slope of the straight line (assuming it is a straight line) is 2.5n.
nucleus
The true nucleus of a comet has only been seen once (P/Halley by spacecraft). From the ground, the star-like nucleus always includes a cloud of dust and gas around the true nucleus. Hence, terms such as stellar condensation and nuclear condensation are often used when a star-like object is seen in the comet's coma. The magnitude of the "nucleus" is denoted m2 and usually isn't of much use because one is really not such what m2 represents. In general, the value of m2 will get fainter as more magnification is applied.
observed magnitude (m1)
The observed magnitude of the comet represents the integrated brightness of the comet's coma or head as seen from Earth. This is normally obtained by comparing the comet's average surface brightness with that of defocused stars (matching the comet's size) of known brightness. Because comets have size (in contrast to stars which are pinpoints of light), a comet of a given brightness will appear less obvious than a star of the same brightness.
opposition
The comet is in the midnight sky on the opposite side of the Earth from the Sun. A perfect opposition, which almost never happens, has the comet 180 degrees away from the Sun.
outburst (in brightness)
An unexpected increase in brightness over a short period of time due to the release of dust and gas into the coma from the nucleus. For a visual observer, the nuclear condensation (a bright spot near the center of the coma) will appear to become star-like and brighter in the comet's coma. Over time (hours - days), the size of the nuclear condensation will increase as the dust moves away from the nucleus. The change in brightness can be as little as half a magnitude and as much as many magnitudes.
perihelion
The point in a comet's orbit that it is closest to the Sun.
perihelion date
The date (and time) the comet reaches perihelion.
perihelion distance
The comet's distance from the Sun, usually expressed in Astronomical Units, at perihelion.
periodic or short-period comets
Any comet with an orbital period of less than 200 years. These comets are indicated by a "P/" before the names. For example, P/Halley is Halley's comet or more properly known as periodic Comet Halley. Recently, the International Astronomical Union has started numbering periodic comets that have been seen at more than one apparition. Thus, Halley's Comet is 1P/Halley and P/de Vico is now known as 122P/de Vico.
position angle (PA)
The PA of a tail or other cometary feature represents the direction on the sky (in degrees from north) toward which it is pointing. Thus, a comet in the morning sky (in the east) that has a tail pointing due west will have a PA of 270 degrees. A comet in with a tail poining toward the south-east will have a PA of 135 degrees. It must be stressed that the determination of the PA of a tail (or other feature) requires plotting it on an atlas and measuring the angle with a protractor. PAs should be measured to at least five degree resolution. It is not possible to look in an eyepiece and accurately estimate the PA of a tail. Also, the determination of PA in the polar regions of the sky is very tricky and may not be intuitive.
solar conjunction
The comet is near the Sun in the sky. Usually, this means that the comet will not be observable from Earth.
tail
The comet's tail is its most distinctive feature. Generally pointing away from the Sun, these appendages come in a variety of shapes and lengths. The lengths can vary from a small fraction of a degree (tails are always measured as the angular length either in degrees or minutes of arc [', 60' = one degree]) to the rare few that cover a significant fraction of the sky.
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Airconvent
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