For the whole month of December, the summer constellations can still be observed in the western sky for the first few hours after sunset. The Summer Triangle asterism is still located fairly high in the sky during the first half of the month, with Vega of the constellation Lyra heading west-northwest, Deneb of the constellation Cygnus closest to the zenith, and Altair of the constellation Aquila is heading west southwest (Figures 1a and 1b). The Summer Triangle continues to dive westward and can be observed lying low near the western horizon in late December as seen from Figure 1c [1].

Figure 1: The view of the night sky at 06:00 P.M. on several days of December 2021 using the Stellarium Application.

Figure 1a. December 1, 2021

Figure 1b. December 15, 2021

Figure 1c. December 30, 2021

The members of the autumn constellations can be notably observed as it gets high in the eastern sky. The view of the observable constellations after sunset in mid-December is shown in Figure 2.

Figure 2. The view of the sky after sunset on mid-December 2021 using the Stellarium application

Cassiopeia, a “W” shaped constellation is going to be very prominent and can be easily seen in the sky due to its unique formation and high position in the north-northwestern sky. When the Big Dipper asterism is not visible in the night sky, Cassiopeia may be used as an alternative to and the position of the north star, Polaris. However, when compared to using the Big Dipper, the result achieved using this method may not be as accurate. The star positioned in the middle of the constellation Cassiopeia can be utilized as a pointer star to lead to the approximate position of Polaris, as shown in Figure 3 [1].

Figure 3. The view of the northern sky on mid-December 2021 at 09:00 P.M. showing how to locate the position of the north star Polaris using the constellation Cassiopeia using the Stellarium application

For the whole month of December, Venus can be observed in the southwestern part of the sky after sunset, however, in late December, Venus may be already difficult to observe as it is just a few degrees above the horizon. Jupiter and Saturn can also be observed in the southwestern portion of the sky after sunset.

Figure 4. The view of the southwestern sky after sunset on 07 December 2021 showing the Moon-Venus Conjunction using the Stellarium application

The thin crescent Moon will pass 1°52' S of Venus on 07 December 2021 at 08:49 A.M. The exact moment of the Moon-Venus Conjunction cannot be observed since it will occur during the day. However, the close pairing will be visible after sunset at around 05:41 P.M. (Figure 4) at an altitude of 30°above the southwestern horizon. The close pairing may be observed until 08:13 P.M. as the pair sinks towards the horizon. By then, the Moon and Venus will be at mag -10.4 and mag -4.7 and are located both in the constellation Sagittarius [2].

On 08 December 2021 at 12:09 A.M., Venus will reach its greatest brightness at mag -4.7 [3].

On 08 December 2021, at 09:49 A.M., the 4-day old Moon will pass 4°11' S of Saturn Again, the exact instance of the crescent Moon-Saturn Conjunction will not be observed as it will occur during the day. However, the view of the conjunction may still be enjoyed after sunset around 05:42 P.M. as the Moon and Saturn will be observed 44° above the southwestern horizon (Figure 5) until the pair sets at around 09:17 P.M. [4]. The Moon and Saturn will be at mag -11.0 and mag 0.5, respectively, and will be both located in the constellation Capricornus [4].

Figure 5. The view of the southwestern sky after sunset on 08 December 2021 showing the Moon-Saturn Conjunction using the Stellarium application

On 09 December 2021, the Moon will be passing 4°28' S of Jupiter at around 02:10 P.M. The close pairing will be visible after sunset at around 05:42 P.M. at about 57° above the southwestern horizon (Figure 6). The Moon and Jupiter will be at mag -11.5 and mag -2.3, respectively, and will be both located in the constellation Capricornus [5].

These Moon-planetary conjunctions can be seen through the naked eye or using a pair of binoculars. Please do note that the separation of the above-mentioned conjunctions is too wide to fit in the field of view of a telescope.

Figure 6. The view of the southwestern sky after sunset on 09 December 2021 showing the Moon-Jupiter Conjunction using the Stellarium application

Several meteor showers can be observed for December. The summary details of the observable meteor showers are presented in Table 1. The first column of the table is the name of the meteor shower, which is usually derived from the constellation where the shower's radiant is located. The radiant of the shower, listed in the second column, is a point in the sky where meteors seem to originate. The position of the meteor shower radiants (enclosed in the green solid circle) of the observable meteor showers for December 2021 during its highest position is presented as follows: Pheonicids (Figure 7), December φ Cassiopeids (Figure 8), Puppid-Velids (Figure 9), Monocerotids (Figure 10), σ-Hydrids (Figure 11), Geminids (Figure 12), Comae Berenicids (Figure 13), December Leonis Minorids (Figure 14), and Ursids (Figure 15).

Meteor Shower Name	Radiant	Active Date	Peak Date	Best Viewed Time	ZHR	Lunar Phase
Pheonicids	Phoenix	28 Nov - 09 Dec	02 Dec	08:00 P.M.	var	Waxing Crescent
December φ Cassiopeids	Andromeda	01 - 08 Dec	06 Dec	09:00 P.M.	var	Waxing Crescent
Puppid-Velids	Vela	01 - 15 Dec	07 Dec	03:00 A.M.	10	Waxing Crescent
Monocerotids	Monoceros	05 - 20 Dec	09 Dec	01:00 A.M.	2	Waxing Crescent
σ-Hydrids	Hydra	03 - 15 Dec	12 Dec	03:00 A.M.	3	Waxing Gibbous
Geminids	Gemini	04 - 17 Dec	14 Dec	02:00 A.M.	120	Waxing Gibbous
Comae Berenicids	Leo	12 - 23 Dec	16 Dec	before dawn	3	Waxing Gibbous
December Leonis Minorids	Leo Minor	05 Dec - 04 Feb	19 Dec	05:00 A.M.	5	Full Moon
Ursids	Ursa Minor	17 - 26 Dec	22 Dec	before dawn	10	Waning Gibbous

The third column shows the period when the meteor shower is active and can be observed while the Peak Date as shown in the fourth column indicates the point in time when the greatest number of meteors can be observed. The Best viewed time, listed in the fifth column, represents the time when the radiant point is at its highest point in the sky. The Zenithal Hourly Rate (ZHR), listed in the sixth column, indicates the number of meteors an observer can see in an hour during the peak activity. However, the computation of ZHR assumes that one is observing in a perfectly dark and clear sky and the radiant is located directly overhead. The number of ZHR decreases under certain circumstances. The last column shows the phase of the Moon during the peak date of the shower. Unfortunately, the waxing gibbous phase of the Moon during the peak of Geminids and Comae Berenicids, the Full Moon phase during the peak of December Leonis Minorids, and the waning gibbous phase of the Moon during Ursids will produce a significant interference in the meteor shower observation throughout the night.

Meteor showers are can be observed through the naked eyed and there is no need to use special equipment such as telescopes or binoculars. To maximize the viewing experience, it is ideal to observe in a dark site away from the city lights under clear and moonless sky conditions.

*(Click image to enlarge view)

Figure 7. The view of the sky during the peak of Pheonicids on 02 December 2021 at 08:00 P.M.	Figure 8. The view of the sky during the peak of December φ Cassiopeids on 06 December 2021 at 09:00 P.M.	Figure 9. The view of the sky during the peak of Puppid-Velids on 07 December 2021 at 03:00 A.M.
Figure 10. The view of the sky during the peak of Monocerotids on 09 December 2021 at 01:00 A.M.	Figure 11. The view of the sky during the peak of σ-Hydrids on 12 December 2021 at 03:00 A.M.	Figure 12. The view of the sky during the peak of Geminids on 14 December 2021 at 02:00 A.M.
Figure 13. The view of the sky during the peak of Comae Berenicids on 16 December 2021 at 02:00 A.M.	Figure 14. The view of the sky during the peak of December Leonis Minorids on 19 December 2021 at 05:00 A.M.	Figure 15. The view of the sky during the peak of Ursids on 22 December 2021 at 02:00 A.M.

On 04 December 2021 from 01:29 P.M. to 05:36 P.M., the Moon will be passing in front of the Sun, thus, creating a rare astronomical phenomenon called Total Solar Eclipse. Unfortunately, the Total Solar Eclipse will only be visible in Antarctica and not in the Philippines [6].

On the 12th of December 2021, another comet named C/2021 A1 (Leonard) will make its closest approach to the Earth at a distance of 0.23 AU. The comet, however, will not be visible during its perigee since it will be below the horizon after sunset. Luckily, there are other chances to see the comet on a different day of the month. The position of Comet C/2021 A1 (Leonard) every other night for the whole month of December 2021 is shown in Table 2 [7].

Date	Constellation	Observable Time	Highest Position in the Sky
01 Dec	Canes Venatici	03:42 A.M. - 05:09 A.M.	37° above NE horizon (at 05:09 A.M.)
03 Dec	Bootes	03:57 A.M. - 05:10 A.M.	34° above NE horizon (at 05:10 A.M.)
05 Dec	Bootes	04:15 A.M. - 05:11 A.M.	29° above NE horizon (at 05:11 A.M.)
07 Dec	Bootes	04:50 A.M. - 05:13 A.M.	23° above NE horizon (at 05:13 A.M.)
09 Dec	Serpens Caput		Not observable
11 Dec	Ophiuchus		Not observable
13 Dec	Serpens Cauda		Not observable
15 Dec	Sagittarius		Not observable
17 Dec	Sagittarius		Not observable
19 Dec	Sagittarius		Not observable
21 Dec	Microscopium	06:28 P.M. - 06:33 P.M.	18° above SW horizon (at 06:28 P.M.)
23 Dec	Microscopium	06:30 P.M. - 06:42 P.M.	19° above SW horizon (at 06:30 P.M.)
25 Dec	Microscopium	06:28 P.M. - 06:42 P.M.	20° above SW horizon (at 06:28 P.M.)
27 Dec	Microscopium	06:30 P.M. - 06:43 P.M.	20° above SW horizon (at 06:30 P.M.)
29 Dec	Piscis Austrinus	06:31 P.M. - 06:39 P.M.	18° above SW horizon (at 06:31 P.M.)
31 Dec	Piscis Austrinus	06:33 P.M. - 06:36 P.M.	18° above SW horizon (at 06:33 P.M.)

The December Solstice will be on 21 December 2021 at 11:59 P.M. By then, the Sun will reach its most southerly point in the sky, in the constellation Capricornus, at a declination of 23.5°S. During the December Solstice, the northern hemisphere will experience the shortest day and this day also marks the first day of winter. Consequently, in the southern hemisphere, this day marks the first day of summer [8].

*All times displayed are in Philippine Standard Time (PhST)

[1] T. U. of Arizona, \Skywatcher's Guide: December 2020 and January 2021." https://flandrau.org/publication/skywatchers-guide-december-2020-and-january-2021, Last accessed on 2021-11-4, 2021.
[2] D. Ford, \In-The-Sky.org Guide to the night sky: Conjunction of the Moon and Venus." https://in-the-sky.org/news.php?id=20211207_20_100, Last accessed on 2021-11-4, 2021.
[3] D. Ford, \In-The-Sky.org Guide to the night sky: Venus at greatest brightness." https://in-the-sky.org/news.php?id=20211207_11_100, Last accessed on 2021-11-4, 2021.
[4] D. Ford, \In-The-Sky.org Guide to the night sky: Conjunction of the Moon and Saturn." https://in-the-sky.org/news.php?id=20211209_20_100, Last accessed on 2021-11-4, 2021.
[5] D. Ford, \In-The-Sky.org Guide to the night sky: Conjunction of the Moon and Jupiter." https://in-the-sky.org/news.php?id=20211208_20_100, Last accessed on 2021-11-4, 2021.
[6] D. Ford, \In-The-Sky.org Guide to the night sky: Total Solar Eclipse." https://in-the-sky.org/news.php?id=20211204_09_100, Last accessed on 2021-11-10, 2021.
[7] D. Ford, \In-The-Sky.org Guide to the night sky: C/2021 A1 (leonard) at perigee." https://in-the-sky.org/news.php?id=20211212_19_100, Last accessed on 2021-11-10, 2021.
[8] D. Ford, \In-The-Sky.org Guide to the night sky: December Solstice." https://in-the-sky.org/news.php?id=20211221_07_100, Last accessed on 2021-11-10, 2021.
Ms. Ma. Rosario C. Ramos, RCE
Chief, SSAS - RDTD
PAGASA - DOST
Diliman, Quezon City
Trunkline: 8284-0800 loc 116, 107
Email add: astronomyph.pagasa@gmail.com

• DECEMBER 2021
• NOVEMBER 2021