Get up before dawn on Sunday, January 7, 2018, to view Mars and Jupiter in conjunction in your southeast sky, with the red planet Mars passing about 0.25o south of the giant planet Jupiter. (For reference, 0.25o is the equivalent of one-half of the moon’s 0.5o diameter). This is the first conjunction of Mars and Jupiter since October 15, 2017; and the next one won’t be until March 20, 2020.
You can easily tell which planet is Jupiter and which planet is Mars because Jupiter is the brighter of the two. In fact, Jupiter beams as the brightest starlike object in the January 2018 morning sky, shining some 20 times more brilliantly than Mars. So look first for Jupiter and seek for that nearby faint red “star,” which is actually the red planet Mars. If you can’t see Mars with the eye alone, aim binoculars at Jupiter to view Mars and Jupiter in the same binocular field together.
In the months ahead, both Mars and Jupiter will be moving away from the star Zubenelgenubi and toward the star Antares. But Mars travels through the constellations of the zodiac at a much faster clip than does slow-plodding Jupiter. So Mars will pass 5o north of Antares on February 11, 2018, whereas Jupiter won’t pass to the north of Antares until December of 2019. In fact, you can use dazzling Jupiter as your guide “star” to Zubenelgenubi, the alpha star in the constellation Libra the Scales, for the most of 2018.
When you contrast the brightness of Mars and Jupiter in January 2018, you might find it hard to believe that Mars will actually be the brighter of these two luminaries in six more months, in July 2018. By the end of the first week of July 2018, Mars will finally catch up with Jupiter in brightness. Then in late July 2018, when Mars comes closest to Earth for the year, Mars will shine most brilliantly in Earth’s sky since 2003. Mars will outshine Jupiter by some 1.8 times in late July/early August 2018.
For a period of about two months, Mars will supplant Jupiter as the fourth brightest celestial body, after the sun, moon and Venus. Mars’ reign as the fourth-brightest celestial body (or third brightest in the nighttime sky, after the moon and Venus) will last from about July 7 to September 7.
Mars has a much greater range of brightness in our sky than Jupiter does. That’s because Mars is about 6.5 times more distant from Earth at its farthest than at its closest. In contrast, Jupiter is only about 1.5 times more distant at its farthest than at it closest.
For instance, when Jupiter comes closest to Earth for the year on May 10, 2018, it’ll only be about 30% closer to Earth, and approximately 1.9 times brighter, than on January 7, 2018. Yet, when Mars comes closest to Earth for the year in late July 2018, it’ll be about 85% closer to Earth, and nearly 50 times brighter, than it is on January 7, 2018.
Watch for the great conjunction of Mars and Jupiter in the predawn/dawn sky on January 7, 2018.