August 2021 is all about three exciting celestial events: the opposition of Saturn, the Perseid meteor shower, and the opposition of Jupiter. While the first two have already occurred, it is time for the opposition of Jupiter that will take place on August 20. Here’s everything you must know about this astronomical event.
1. What is an opposition?
In celestial mechanics, a planetary opposition occurs when the Earth lies between the Sun and the planet, with Earth being on the same side of the planet, as shown below.
Thus on August 20, Sun, Earth, and Jupiter will almost align, with Earth in the middle. This optimal positioning occurs when Jupiter is almost directly opposite the Sun in the sky. Since the Sun reaches its greatest distance below the horizon at midnight, the point opposite to it is the highest in the sky at the same time.
At around the same time that Jupiter passes opposition, it also makes its closest approach to the Earth – termed its perigee – making it appear at its brightest and largest. Jupiter’s orbit lies at around 4.2 AU from the Sun, and hence the angular size varies to some extent over the course of conjunction, and opposition, as compared to Saturn that lies further away at 9.2 AU.
2. How close will be Jupiter?
On August 20, Jupiter will be at a distance of about 600 million km (4.01 AU). That’s about 28.5 million km closer than the average distance from Earth. However, even at its closest approach to the Earth, it is impossible to distinguish it as more than a star-like point of light with the naked eye, though a good telescope is sufficient to reveal its four Galilean moons (Ganymede, Io, Europa, and Callisto).
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3. How to see Jupiter and its moons?
Opposition marks the middle of the best time of year to see a planet. The planet will be visible shortly after the sunset, around 20:00. For observers in the northern hemisphere, Jupiter will rise in the southeast direction in the constellation of Capricornus. It will reach its highest point in the sky at 00:30, 47° above the southern horizon. It will become inaccessible around 05:17 when it sinks below 8° above the south-western horizon. The apparent mag of Jupiter will be -2.9, making it the brightest speck of light throughout the night. You can use these space apps to locate Jupiter according to your place quickly.
The apparent magnitude of a celestial object is the measure of its apparent brightness. The smaller the number, the brighter the object. Unaided human eyes can see up to +6.0.
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