Observe the sky: December 2025
December bids farewell to this year 2025, in which we witnessed a spectacular partial solar eclipse and enjoyed your presence at two lunar observations. But December is not only about endings—it’s also about beginnings. This month we embark on our ‘H-alpha’ adventure. We will carry out a different kind of solar observation, one where we’ll be able to marvel at solar flares, see structures on the Sun’s surface like never before, and witness real-time changes in the Sun itself. Are you ready to join us? Let’s dive in!
| Day | Time | Event |
|---|---|---|
| 4 | 12:11 | Moon at perigee ( 357024 km) |
| 5 | 00:17 | Full Moon |
| 7 | 17:54 | Moon in conjunction with Jupiter 3.63° N of Jupiter |
| 7 | 22:30 | Mercury greatest western elongation ( 20.73°) |
| 11 | 21:54 | Last Quarter |
| 17 | 05:29 | Saturn at quadrature |
| 17 | 07:21 | Moon at apogee ( 406306 km) |
| 18 | 11:04 | Moon in conjunction with Mercury 6.09° S of Mercury |
| 19 | 17:23 | Moon in conjunction with Venus 4.87° S of Venus |
| 20 | 02:47 | New Moon |
| 20 | 13:46 | Moon in conjunction with Mars 3.82° S of Mars |
| 21 | 01:48 | Neptune at quadrature |
| 21 | 15:57 | Solstice |
| 27 | 01:43 | Moon in conjunction with Saturn 3.57° N of Saturn |
| 27 | 20:12 | First Quarter |
In December we encounter the so-called Winter Solstice, specifically on December 21 at 15:57 local time in Granada. This day marks the lowest daily maximum height of the Sun in the entire year—in other words, with the Sun so low on the horizon, it doesn’t warm much. In fact, it’s worth noting that Earth is currently very close to its perihelion (the closest position to the Sun in its orbit). However, the tilt of Earth’s rotational axis relative to its orbit around the Sun makes us in the Northern Hemisphere see it so low and for so few hours, giving rise to the typical cold temperatures of winter. The situation is very different in the Southern Hemisphere, where December actually brings the start of summer. Christmas in a swimsuit feels so strange to me!
But what does the winter solstice really mean? At the moment of the solstice, the Sun reaches its minimum declination (-23.43º), coinciding with the tilt angle between Earth’s orbit around the Sun and its axis of rotation. On that day, in places south of latitude 66.57ºS, the Sun will not set—it will never go below the horizon. Conversely, in places north of 66.57ºN, the Sun will not rise above the horizon. In all other locations where the Sun does rise and set, it will do so at its southernmost point. No other day will it rise further south. In the Northern Hemisphere, the Sun reaches its lowest altitude, and from then on it begins to climb higher above the horizon. Thus, on December 21 the Sun will reach its minimum height in Granada at noon, about ~29º. This means that in Spain (and across the Northern Hemisphere) shadows reach their maximum length that day. Etymologically, Solstice means “Sun standing still.” While in September or October the Sun was losing (or gaining, if you’re in the Southern Hemisphere) altitude quickly, it has been “slowing down” until today, when it begins its gradual “ascent.” Moreover, December 21 will be the day of the year with the fewest hours of sunlight. In Granada (sunrise: 8:26, sunset: 18:05), that’s 9 hours and 39 minutes. However, it is neither the latest sunrise (January 6) nor the earliest sunset (December 5).
In terms of planets, this is clearly the month of the outer planets (those with orbits beyond Mars). Neither Mercury, Venus, nor Mars will be easily observable these days. However, the main protagonists will be Saturn (still with its tilted rings) and Jupiter, which is gradually moving into a more favorable position for observation. As every month, I’m sharing a table with the main events involving Jupiter’s satellites (Jupiter table). I would highlight the early hours of December 6–7, the afternoon-evening-early morning of December 15–16, and the night of December 30.
As for the stars, little by little we are leaving behind the autumn sky (though it is in December when the transition from one season to another truly takes place), and beginning to enjoy the winter sky, dominated by bright stars and a spectacular Milky Way. Moreover, because night falls so early, we can still enjoy, at the start of the evening, objects characteristic of the summer sky. These are good nights to recall our summer adventures and to begin observing the winter constellations.
From a more down-to-earth perspective, here are also the main sightings of the International Space Station from Granada (ISS table) . We highlight the passes on December 3, 15, and 31, with magnitudes of -3.7, -3.3, and -3.8 respectively, with the last two visible at dawn.
Thus we conclude this brief summary of the astronomical events we will enjoy during December—a month in which we will inaugurate our brand-new H-alpha solar telescope. As always, we remind you that we are open to suggestions on our Telegram channel, where you can also stay up to date with activities, receive timely alerts about easily observable astronomical events, and much more. Here’s the link! We look forward to seeing you soon at our activities. I hope you have a good end and beginning of the year, and that you recharge plenty of energy for the exams that are starting to feel a little too close.
Best of luck—and Clear Skies!
