Planets are very much in the fore these days, especially with the two largest — Jupiter and Saturn — now putting on a show as prominent evening luminaries. This week, both planets appear at their very best, with Jupiter having just arrived at opposition this past Tuesday (July 14) and Saturn to reach its own opposition on Monday (July 20).
On Dec. 21, these two planets will be in conjunction — meaning they'll share the same celestial longitude while making a close approach in the night sky — for the first time since the year 2000. It takes Saturn almost 30 years to make one trip around the sun, while it only takes Jupiter about 12 years to complete one solar revolution. As a result, Jupiter appears to overhaul Saturn at intervals of roughly 20 years.
Let's check out both of these giant worlds, which will remain prominent objects in our evening sky through the balance of this year. Saturn and Jupiter provide telescope users with a feast of features. Saturn of course has its splendid ring system and Jupiter can boast a restless atmosphere and retinue of bright moons.
As we just noted, Jupiter arrived at that point in the sky directly opposite to the sun, called "opposition," on Tuesday (July 14). If the planets' paths around the sun were true circles, this would also coincide with Earth's closest approach to Jupiter, 384.8 million miles (619.1 million kilometers). However, because the planets' orbits are slightly oblong, that actually occurred the following day, on Wednesday (July 15).
Opposition occurs when the Earth, moving faster in its orbit than Jupiter, overtakes it. (This is true for any of the outer planets.) From now on, we'll leave Jupiter behind, catching up with the planet again on Aug. 19, 2021. Jupiter is also currently moving closer to the sun in its own elliptical path and will reach its closest point to the sun, 460 million miles (740 million km), at its perihelion point on Jan. 20, 2023.
Jupiter, the largest planet in the solar system, shines as a brilliant silvery "star" to the upper left of the famous Teapot asterism of Sagittarius, low in the east-southeast sky as dusk arrives and will now appear to climb higher in the evening sky in the weeks to come.
Currently, this giant planet is ready for telescopic observing by 10:45 p.m. local time, when it will stand roughly one-quarter of the way up from the horizon to the point directly overhead, called the zenith. It reaches its highest position in the south around 12:30 a.m. and is heading toward its setting in the west-southwest during dawn.
With a diameter of 88,800 miles (143,000 km), Jupiter is a colossal ball of hydrogen and helium without a solid surface. It has a rocky core encased in a thick mantle of metallic hydrogen enveloped in a massive atmospheric cloak of multi-colored clouds of ammonium hydrosulphide.
In a strange sense, Jupiter might even be referred to as a "stillborn star," for it has the makings (mostly hydrogen) if not the mass of a stellar body. Its relative smallness, however, prevents the initiation of the nuclear processes that could have turned it into a full-fledged star. Had this been the case, we would have the distinction of living within a binary star system.
Jupiter is the most consistently interesting object in the solar system after the moon and the sun and has always held a special place in the hearts of telescope viewers.
The smallest telescope — even steadily held 7-power binoculars — show Jupiter as a tiny disc, while a medium-size telescope reveals numerous dark belts, light zones and a wealth of festoons, garlands, ovals and other features extending here and there.
But Jupiter's greatest telescopic treasure are its four Galilean satellites that run a merry chase with each other around the planet (in all Jupiter has 79 confirmed moons), changing their respective positions from hour to hour and night to night.
Typically, at least two or three Galilean moons are visible at any given moment. The four — Io (I), Europa (II), Ganymede (III) and Callisto (IV), numbered by the order in which they were named — are all larger than Earth's moon. They can be followed for hours as they speed in front of Jupiter (throwing their shadows on the planet), vanish behind its giant disk or plunge into its shadow. They appear as tiny stars nearly in line and changing their places in the line as they revolve around the planet in orbits nearly edgewise to us.
On July 22, for example, we would see all four satellites on one side of Jupiter. Moving outward from Jupiter, they will also be, interestingly, in numerical order: Io, Europa, Ganymede and Callisto.
source : https://www.space.com/jupiter-and-saturn-opposition-july-2020.html