1.
Comet Encke
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Comet Encke or Enckes Comet is a periodic comet that completes an orbit of the Sun once every 3.3 years. Like most comets, it has a low albedo, reflecting only 4. 6% of the light it receives. The diameter of the nucleus of Enckes Comet is 4.8 km, as its official designation implies, Enckes Comet was the first periodic comet discovered after Halleys Comet. It was independently observed by astronomers, the second being Caroline Herschel in 1795. Its orbit was calculated by Johann Franz Encke, who through laborious calculations was able to link observations of comets in 1786,1795,1805 and 1818 to the same object. In 1819 he published his conclusions in the journal Correspondance astronomique and it was recovered by Carl Ludwig Christian Rümker at Parramatta Observatory on 2 June 1822. Comets are in orbits that evolve over time due to perturbations. Given Enckes low orbital inclination near the ecliptic and brief orbital period of 3 years, Enckes orbit gets as close as 0.17309 AU to Earth. On 4 July 1997, Encke passed 0.19 AU from Earth, on 18 November 2013, it passed 0.02496 AU from Mercury. Close approaches to Earth usually occur every 33 years, the failed CONTOUR mission was launched to study this comet, and also Schwassmann-Wachmann 3. On April 20,2007, STEREO-A observed the tail of Comet Encke to be torn off by magnetic field disturbances caused by a coronal mass ejection. The tail grew back due to the shedding of dust. Comet Encke is believed to be the originator of several related meteor showers known as the Taurids, a shower has similarly been reported affecting Mercury. Near-Earth object 2004 TG10 may be a fragment of Encke, measurements on board the NASA satellite MESSENGER have revealed Encke may contribute to seasonal meteor showers on Mercury. The Mercury Atmospheric and Surface Composition Spectrometer instrument discovered seasonal surges of calcium since the probe began orbiting the planet in March 2011. The spikes in calcium levels are thought to originate from small dust particles hitting the planet, however, the general background of interplanetary dust in the inner Solar System cannot, by itself, account for the periodic spikes in calcium. This suggests a source of additional dust, for example. More than one theory has associated Enckes Comet with impacts of cometary material on Earth, the Tunguska event of 1908, probably caused by the impact of a cometary body, has also been postulated by Czechoslovakian astronomer Ľubor Kresák as a fragment of Comet Encke

2.
94P/Russell
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94P/Russell 4 is a periodic comet in the Solar System. It fits the definition of an Encke-type comet with and it was discovered by Kenneth S. Rusell on photographic plates taken by M. Hawkins on March 7,1984. In the discovery images, Russell estimated that the comet had an apparent magnitude of 13, in the year of discovery, the comet had come to perihelion in January 1984. With an aphelion of 4.7 AU, comet 94P currently has an orbit contained completely inside of the orbit of Jupiter, in July 1995, 94P was estimated to have a radius of about 2.6 km with an absolute magnitude of 15.1. It may have a very elongated nucleus with a ratio of a/b >=3

3.
Semi-major and semi-minor axes
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In geometry, the major axis of an ellipse is its longest diameter, a line segment that runs through the center and both foci, with ends at the widest points of the perimeter. The semi-major axis is one half of the axis, and thus runs from the centre, through a focus. Essentially, it is the radius of an orbit at the two most distant points. For the special case of a circle, the axis is the radius. One can think of the axis as an ellipses long radius. The semi-major axis of a hyperbola is, depending on the convention, thus it is the distance from the center to either vertex of the hyperbola. A parabola can be obtained as the limit of a sequence of ellipses where one focus is fixed as the other is allowed to move arbitrarily far away in one direction. Thus a and b tend to infinity, a faster than b, the semi-minor axis is a line segment associated with most conic sections that is at right angles with the semi-major axis and has one end at the center of the conic section. It is one of the axes of symmetry for the curve, in an ellipse, the one, in a hyperbola. The semi-major axis is the value of the maximum and minimum distances r max and r min of the ellipse from a focus — that is. In astronomy these extreme points are called apsis, the semi-minor axis of an ellipse is the geometric mean of these distances, b = r max r min. The eccentricity of an ellipse is defined as e =1 − b 2 a 2 so r min = a, r max = a. Now consider the equation in polar coordinates, with one focus at the origin, the mean value of r = ℓ / and r = ℓ /, for θ = π and θ =0 is a = ℓ1 − e 2. In an ellipse, the axis is the geometric mean of the distance from the center to either focus. The semi-minor axis of an ellipse runs from the center of the ellipse to the edge of the ellipse, the semi-minor axis is half of the minor axis. The minor axis is the longest line segment perpendicular to the axis that connects two points on the ellipses edge. The semi-minor axis b is related to the axis a through the eccentricity e. A parabola can be obtained as the limit of a sequence of ellipses where one focus is fixed as the other is allowed to move arbitrarily far away in one direction