Dave Gary
Comet C/2021 A1 Leonard 10/16/2021 ~06:30
The following description taken from Sky&Telescope weekly update:
Senior research specialist Greg Leonard at Mt. Lemmon Observatory discovered the 19th-magnitude speck on January 3, 2021, exactly one year before perihelion. Orbital calculations revealed that the object had spent the last 35,000 years wending its way sunward after reaching aphelion at the chilling distance of around 3,500 a.u. Comet Leonard will pass nearest the Sun again on January 3, 2022, at 0.62 a.u. Two weeks before that on December 12th, it makes its closest approach to Earth at 34.9 million kilometers.
Magnitude estimates for comets are a little different than magnitude estimates for other celestial objects; for example, stars, planets, Moon, Sun. This description, taken from the web page "Light Curve of Hale-Bopp", makes these types of estimates much more clear:
General Concepts
The first step in obtaining a light curve of a comet is to obtain a series of estimates of the comet's brightness, the integrated brightness of the comet's head or coma. For relatively bright comets like Hale-Bopp, these estimates are typically made by (amateur) visual observers, who compare the comet's head to defocused stars of known brightness. The stars are defocused because the comet's head has size whereas, the stars are points of light. An observer defocuses the stars until they match the size of the comet's head. Then the relative brightness of the comet is determined by comparing the comet's brightness with the defocused comparison stars. The result is a magnitude estimate of the comet, the observed magnitude. [The brightness of stars is also expressed in magnitudes. The smaller the magnitude, the brighter the object. The brightest star in the sky, Sirius, is magnitude -1.5. Polaris, the pole star is about magnitude 2. Jupiter is about -2, and the faintest stars that can typically be seen with the naked eye (in very dark skies away from the city) are roughly magnitude 6-7. In contrast, the full Moon is about magnitude -13 and the Sun is approximately -26! A one magnitude change amounts to a factor of 2.5 in brightness. A five magnitude change is a factor of 100 in brightness.
A comet's brightness variation with respect to its distance from the Sun is often represented by a "power-law" formula:
observed magnitude = absolute magnitude + 5 log (delta) + 2.5 n log (r)
where delta is the Earth-comet (geocentric) distance in astronomical units (AU), r is the Sun-comet (heliocentric) distance in AU and n is the "power-law exponent." One AU is equal to the Earth-Sun distance - about 93 million miles or 150 million kilometers. In this equation:
The observed magnitude is what one sees when observing the comet from Earth.
The absolute magnitude is a quantity that is suppose to tell us something about the intrinsic brightness of the comet. For Hale-Bopp this number indicates an intrinsically bright comet. Unfortunately, this parameter really isn't absolute...it can change during the apparition. (In fact it has already changed during the apparition.)
The 5 log (delta) part of the equation takes into account how the comet's brightness changes with changing Earth-comet distance (delta). This assumes an inverse-squared relationship. That is, if the distance between the Earth and comet increases by a factor of 2, the brightness will decrease by a factor of (2) squared or four. A factor of 3 in distance results in a change of 9 times in brightness. This is a basic property of all light sources.
The last term, 2.5 n log (r), represents the change in brightness due to the comet's changing Sun-comet distance. If the comet's brightness varied by pure reflection, n=2 and thus, 5 log (r) or the inverse-power relationship. However, comets produce both dust and gas, thus providing more stuff to reflect light. Also, the Sun causes the gas to fluoresce. Because of this, a comet's brightness typically changes more rapidly with changing Sun-comet distance. The value of n defines how rapid this change is. For n=4, an overall average for comets, reducing the Sun-comet distance by a factor of 2 increases the brightness by 16 times (compared with the four times for pure reflection).
The two terms of most interest are the absolute magnitude and n. They determine the intrinsic variation of a comet's brightness.
Magnitude estimates at this time put this comet at ~+12.2. I took no defocused images of surrounding stars and made an attempt to calculate the magnitude of this comet myself. I'll take their word for it concerning magnitude estimates. Besides, this would require effort. Guys, you know me, I try not to do anything that requires effort.
I took this image at around 06:30 from the Driveway Observatory in St. George, Utah. I'll keep watching this one.
Comet C/2021 A1 Leonard 10/16/2021 ~06:30
The following description taken from Sky&Telescope weekly update:
Senior research specialist Greg Leonard at Mt. Lemmon Observatory discovered the 19th-magnitude speck on January 3, 2021, exactly one year before perihelion. Orbital calculations revealed that the object had spent the last 35,000 years wending its way sunward after reaching aphelion at the chilling distance of around 3,500 a.u. Comet Leonard will pass nearest the Sun again on January 3, 2022, at 0.62 a.u. Two weeks before that on December 12th, it makes its closest approach to Earth at 34.9 million kilometers.
Magnitude estimates for comets are a little different than magnitude estimates for other celestial objects; for example, stars, planets, Moon, Sun. This description, taken from the web page "Light Curve of Hale-Bopp", makes these types of estimates much more clear:
General Concepts
The first step in obtaining a light curve of a comet is to obtain a series of estimates of the comet's brightness, the integrated brightness of the comet's head or coma. For relatively bright comets like Hale-Bopp, these estimates are typically made by (amateur) visual observers, who compare the comet's head to defocused stars of known brightness. The stars are defocused because the comet's head has size whereas, the stars are points of light. An observer defocuses the stars until they match the size of the comet's head. Then the relative brightness of the comet is determined by comparing the comet's brightness with the defocused comparison stars. The result is a magnitude estimate of the comet, the observed magnitude. [The brightness of stars is also expressed in magnitudes. The smaller the magnitude, the brighter the object. The brightest star in the sky, Sirius, is magnitude -1.5. Polaris, the pole star is about magnitude 2. Jupiter is about -2, and the faintest stars that can typically be seen with the naked eye (in very dark skies away from the city) are roughly magnitude 6-7. In contrast, the full Moon is about magnitude -13 and the Sun is approximately -26! A one magnitude change amounts to a factor of 2.5 in brightness. A five magnitude change is a factor of 100 in brightness.
A comet's brightness variation with respect to its distance from the Sun is often represented by a "power-law" formula:
observed magnitude = absolute magnitude + 5 log (delta) + 2.5 n log (r)
where delta is the Earth-comet (geocentric) distance in astronomical units (AU), r is the Sun-comet (heliocentric) distance in AU and n is the "power-law exponent." One AU is equal to the Earth-Sun distance - about 93 million miles or 150 million kilometers. In this equation:
The observed magnitude is what one sees when observing the comet from Earth.
The absolute magnitude is a quantity that is suppose to tell us something about the intrinsic brightness of the comet. For Hale-Bopp this number indicates an intrinsically bright comet. Unfortunately, this parameter really isn't absolute...it can change during the apparition. (In fact it has already changed during the apparition.)
The 5 log (delta) part of the equation takes into account how the comet's brightness changes with changing Earth-comet distance (delta). This assumes an inverse-squared relationship. That is, if the distance between the Earth and comet increases by a factor of 2, the brightness will decrease by a factor of (2) squared or four. A factor of 3 in distance results in a change of 9 times in brightness. This is a basic property of all light sources.
The last term, 2.5 n log (r), represents the change in brightness due to the comet's changing Sun-comet distance. If the comet's brightness varied by pure reflection, n=2 and thus, 5 log (r) or the inverse-power relationship. However, comets produce both dust and gas, thus providing more stuff to reflect light. Also, the Sun causes the gas to fluoresce. Because of this, a comet's brightness typically changes more rapidly with changing Sun-comet distance. The value of n defines how rapid this change is. For n=4, an overall average for comets, reducing the Sun-comet distance by a factor of 2 increases the brightness by 16 times (compared with the four times for pure reflection).
The two terms of most interest are the absolute magnitude and n. They determine the intrinsic variation of a comet's brightness.
Magnitude estimates at this time put this comet at ~+12.2. I took no defocused images of surrounding stars and made an attempt to calculate the magnitude of this comet myself. I'll take their word for it concerning magnitude estimates. Besides, this would require effort. Guys, you know me, I try not to do anything that requires effort.
I took this image at around 06:30 from the Driveway Observatory in St. George, Utah. I'll keep watching this one.