Big Jove not Done for the Season

By Avid planet watcher – Milan B

It’s mid September. It’s been more than 3 months since Jupiter reached opposition in early June. It was a memorable summer for many Jupiter observers like myself despite fewer than usual clear summer nights and the lower than usual position of Jupiter for northern observers.

As Big Jove is heading towards the deep winter sleep, barely hanging in the southwestern sky after the sunset, one may ask: when will this apparition of Jupiter end, isn’t it almost over?

Observe Jupiter Image
Brilliant Jupiter or Big Jove currently appears low in the southwestern sky just after sunset. Image credit: National Geographic Magazine.

Well, it’s true that the big gas giant lies very low in the sky, waiting for the fast approaching Sun to catch up from the west, but the season finale is being delayed by an unexpected gift from the Sun itself.

After spending nearly six months decorating the northern hemisphere our local star is quickly descending towards the Celestial Equator and then into the Southern Hemisphere taking with it a big chink of daylight for northern mid-latitude observers, almost 4 minutes every day. The depletion of the daylight hours is especially visible in the evenings — only in the month of September does the sunset times shift by more than one hour (earlier in the day) for observers at 49° North.

The Sun is moving eastward throughout the year, while Jupiter switched from retrograde (westward) to direct (eastward) motion around the 11th of August as it started galloping through the southern zodiac constellations. This results in Jupiter setting times (which are occurring earlier each day) slightly delayed compared to the stationary stars. For stars (which rise and set) the difference between two consecutive star-rises or star-sets is about 4 minutes – the difference between the solar day and the sidereal day on Earth. 

For Jupiter this difference is already much lower than 4 minutes for all of September and it is being further reduced as Jupiter is heading towards the superior conjunction with the Sun in December this year. However, the solar contribution to this delay (the pace of the shortening of the daylight hours) will decay slowly as we enter the autumn months and will completely stop at Winter Solstice. 

The peak of this unusual delay will be in the period between October 3rd and October 9th as shown in the table below. During this week we will lose only 9 minutes of potential observing time for Jupiter.

Table 1) A combination of Jupiter’s accelerating eastward motion in the sky and the rapid descent of the Sun in northern skies results in extending this year’s Jupiter observing season by a few weeks.

Jupiter will be visible for at least another two months, sinking ever lower in the SW sky, before the Solar System’s largest planet gets lost in the December twilight. So, please go out and enjoy the big Jove’s apparition while it lasts.

Could celestial geometry arrange a completely opposite phenomenon from the one mentioned above? Stay tuned for the accelerated season ending of the brightest star in the sky – Sirius. It will happen, you might have guessed, exactly six months from now.