Abstract
An analysis is presented of the visual observations of the Geminids gathered in December 2018. In 2017 there were two main peaks in activity of the Geminids, a first at solar longitude 261.9 ° and the second at solar longitude 262.2 [1 & 2]. The first peak is also seen in de 2018 data although slightly later in time. The second peak was not observed as this time was not covered by observations in 2018.

Introduction
The year 2018 was an excellent year to observe the Geminids. With a Moon in its First Quarter on December 15, this only meant some moonlight in the evening sky. This article gives the results of the analysis of the Geminid activity in 2018, based on visual observations. In addition, this analysis is compared with the analysis from 2017 [1 & 2]. The 2018 analysis is interesting because in the series of previous years 1994-2002 and 2010, the Moon still disturbed considerably. In that respect, 2018 is the best year in terms of moonlight compared to the years mentioned. In 2026 the Moon will not disturb the observations at all and we can map this series of years even better! See also table 1. In this article, however, no comparison with this series of years as done in [3 & 4]. The author returns to this aspect separately in a forthcoming article.

Table1. Moon set during the Geminid campaigns of
1994-2002-2010-2018-2026 (situation in the Netherlands!).

Collecting the data
All data was collected during spring 2019. Most observations were gathered at the IMO website (www.imo.net), but the author also received observations from observers who did not report to IMO. Only data was used that met the following requirements:
• Only observations from observers with a known Cp were used;
• Only observations with limiting magnitudes 5.9 or higher were used;
• Only observations with a radiant height of at least 25 degrees were used;
• Extreme outliers were removed.

Population index r
The population index r could be calculated for several nights. The magnitude distributions of observers with a good Cp determination were examined. The rule here is: the difference between the average limiting magnitude and the average magnitude of the Geminids may not be greater than 4.5 magnitude. In the end, more than 5100 Geminids could be used to determine the population index r. Table 2 and Figure 1 give the result.
Table 2 shows that only the period 11 to 15 December provided enough data to calculate reliable population index r values. The population index r on the magnitude range [-1; 5] was finally used in the final ZHR calculations.

Table 2. Population index r Geminids 2018.

Figure 1. Population index r[-2;5] and r[-1;5] from the Geminids between December 11, 2019 00h00m UT and December 15, 2019 06h00m UT. The low R value at λʘ 262.0 indicates that a lower population index r also occurs after the first peak (more bright meteors). This is interesting enough to look at in the coming years to see if it is an annual phenomenon.

Zenithal Hourly Rate
The ZHRs are always calculated according to the method of Peter Jenniskens as described in [4, 5]

ZHR = n x (sin h)-γ x r (6.5-LM) x Cp-1/ Teff (1)

However, the radiant height correction factor “gamma” is set to 1.0 instead of 1.4. After all the data that met the criteria mentioned had been obtained, 8826 Geminids remained for processing. For the nights up to December 11, all ZHR values per night were calculated (weighted averages); for the night 11/12 December from the periods before and after 00h UT, for the nights 12/13, 13/14 and 14/15 December the ZHR could be determined per hour over Europe and partly for America. The results are presented in table 3 and figure 2.

Table 3. ZHR of the Geminids in 2018.

Figure 2. Zenithal Hourly Rate of the Geminids between December 3 and 17, 2018.

Figure 2 shows that the build-up of the Zenithal Hourly Rate is rather flat between December 3 and 10, 2018, with the ZHR values ranging between 5 and 10. The night of December 11/12 gives ZHR values of 20-25 for Europe. The night December 12/13, 2018 in Europe gives ZHR values between 30-70 and over America the ZHRs varied between 50 and 70. The maximum night December 13/14, 2018 had ZHRs between 70 and 110 over Europe and above America the ZHRs rose to 100-130. The night of 14/15 December gives ZHRs (above Europe) decreasing from 60-80 to 20-30. Above America ZHRs varied between 40 and 30. One night later, the Geminid activity is almost gone. Below we zoom in on the nights 12/13, 13/14 and 14/15 December 2018 respectively, because there are quite a few comments to make here and there. Figure 3 below gives a more detailed look of the Geminids ZHR in the period from 12 to 15 December 2018.

Figure 3. ZHR of the Geminids between 12 and 15 December 2018

12/13 December 2018 (Europe and northern America)

Figure 4. Detailed view of Geminids ZHR in the period from
December 12, 2018 20h00m UT to December 13, 11h00m UT.

For this night the author used 15-30 minutes counts to obtain a weighted average of the ZHR. Figure 4 shows that this year two impressive sub maxima were observed above Europe and America. A first maximum took place above Europe on December 13 around 02h30m UT with a ZHR of 70 ~ 5. This is followed by a decrease in activity to a ZHR 38 ~ 3 around 4h30m UT. Then a second maximum follows around 7h45m UT with a ZHR of 75 ± 11 best visible above the eastern part of North America. After this peak the activity decreased back to ZHR 40-50. It is striking that both peaks show a clear increase and decrease that fall within the error margins. So, in other words these are two real peaks. Unfortunately, there is not enough data available from Asia so that we cannot say anything about the activity of the Geminids there.

13/14 December 2018: Europe and America

Figure 5. Detailed view of the Geminid activity between
December 13, 2018 19h00m UT and December 14, 2018 11h00m UT.

For this night, the ZHR was obtained from 10-20 minutes counts and a weighted average of the ZHR values was calculated. The first good data of the maximum night above Europe is available from 19h00m UT. Figure 5 shows the result. We see an increasing ZHR from 62 ± 12 around λʘ 261.48 (= December 12, 2018 19h36m UT) to ZHR 107 ~ around λʘ 261.74 (= December 14, 2018 1h30m UT). After that, the ZHR decreased to 80, to rise then again at the end of the night in Europe to ZHR 107 ± 8 around λʘ 261.94 (= December 14, 2018 06h20m UT). The first data point from America is at λʘ 261.98 (= 14 December 2018 07h10m UT) with a ZHR of 126. It should be noted that all data points from North America come from only 1 or 2 observers.

Figure 6. Comparison between the ZHR of the Geminids in 2017 and in 2018.

The data from 2018 was then compared with data from 2017 [2]. See figure 6 for the result. This is to see whether the two peaks found in 2017 were again visible in the observational data.
If we look very closely at figure 6 at the first maximum of 2017 at λʘ 261.9, it will fall slightly later in 2018. The ZHR in 2017 was 135 ± compared to ZHR 125 ± 9 in 2018. Subsequently, a decrease in activity was observed in both years. However, in 2017 the decrease was much sharper and much deeper than in 2018, although the data did not completely overlap in this period. A cautious conclusion is that the first maximum from 2017 is well observed in 2018, and that the second maximum was not observed in 2018.

14/15 December 2019 (Europe and America)

Figure 7. Details of the ZHRs of the Geminids for 14/15 December 2018

As usual, a nice decreasing activity of the Geminids during the night December 14/15, 2018. See figure 7. With ZHRs up to 80 ± 13 at the start of the night in Europe, at the end of the night the ZHR is close to 20 ± 4. Figure 8 shows a comparison with 2017. This shows that the 2017 data is reasonably consistent with that of 2018.

 

 

Figure 8. Comparison between the ZHR of the Geminids from
the night of December 14/15, 2017 and 2018.

Outlook for the 1st and 2nd Geminid maximum in 2020
In table 4 (below) the author made predictions for both maxima of the Geminids. It has been assumed that the two maxima are always a recurring phenomenon. Furthermore, the first maximum is based on the value found in 2018 at λʘ 262.95. The first peak of 2017 was slightly earlier than in 2018 . The second maximum is based on λʘ 262.2. Therefore, this year is important, because we can more or less observe the two maxima from Europe for the first time. Between the maxima we will probably have the well-known dip in activity that went much deeper in 2017 than in 2018. So, be aware of some disappointing activity between both peaks (ZHR 60-100)! That observations of both peaks were not possible in 1994, 2002 and 2010 was because the 1st maximum always took place too early, so it was still not yet dark enough or the radiant height was too low.

Table 4. Prospects for the Geminids in 2020 from different locations. In the column λʘ in blue the expected solar longitudes for both maxima. At right 5 columns with the radiant elevations at specific locations. The blue numbers in bold indicate the maximum radiant height at that specific location. The good observing period is indicated in light blue, considering radiant rise and / or start / end of the twilight.

If we look at Table 4, there are advantages and some disadvantages for all locations. Incidentally, it is worth mentioning that in the evening during the Geminid maximum (period) a beautiful conjunction is visible between the planets Jupiter and Saturn. Also, the planet Venus will also be visible in the early morning hours.

From the Netherlands, we can observe a nice long period and both maxima will be observable. During the 1st maximum the radiant height is around 18-26 degrees. The 2nd maximum takes place during maximum radiant height. After that we can still see the decreasing branch with the bright Geminids for 4 to 5 hours! The disadvantage is that the chance to have a clear night in December in the Netherlands is only 10%.

From France (Provence) the same story, around the 1st maximum the radiant height is about 25 degrees, around the 2nd maximum the radiant height culminates at 79 degrees. However, the chances for clear weather are also not so great there (20-30%), sometimes with strong local differences.

From Portugal, the 1st maximum is barely visible because of a very low radiant height (only 10 degrees). The advantage is that after the 2nd maximum you can still enjoy the bright Geminids for 5 to 6 hours. In Portugal there is a better chance for clear weather than in France. In Portugal there is a 40-50% chance to have a clear night in December.

From Tenerife, the 1st maximum is not visible because the radiant is below the horizon at that moment, but the 2nd maximum is good with the big advantage that the decline in activity with many bright Geminids for 5 to 6 hours can be observed. Weather conditions are better than in southern France and Portugal with 50-70% or more chance for a clear night.

From Oman we can observe both maxima, with the 1st maximum there being the best perceptible in terms of radiant height (42-54 degrees) compared to the 4 other locations. The 2nd maximum can also be seen there with a reasonable radiant height, but the radiant height is the lowest compared to the 4 other locations (also 54 degrees high). The disadvantage is that the decreasing branch of the second maximum with bright Geminids can only be observed for two hours with a lower radiant height. Weather wise, little can go wrong here.

Acknowledgment
The author thanks Paul Roggemans, Michel Vandeputte and Carl Johannink for critically reading this article and for their advises. In addition, a huge word of thanks to all the observers who have observed the Geminids of 2018. The author knows from experience how difficult that can be in terms of finding clear weather and/or cold weather conditions. It takes a lot of perseverance! Hope you are all active during the coming Geminid returns! The observers in 2018 were:

Alexandre Amorin, Pierre Bader, Benitez Sanchez Orlando, David Buzgo, Tim Cooper, Kelly de Lima Gleici, Katie Demetriou, Mayuresh Desai, John Drummond, Garry Dymond, Mohammad Iman Fotouhi, Kai Gaarder, Aldrin Gabuya, Carl Hergenrother, Gabriel Hickel, Glenn Hughes, Gerardo Jiménez López, Carl Johannink, Hansub Jung, Károly Jónás, Javor Kac, Omri Katz, Shubham Kawabe, Kajal (FC) Kesare, Roman Kostenko, Greet Lembregts, Jan Lembregts, Anna Levin, Ole Lit,Vincent Marik, Adam Marsh, Jemma Marsh, Pierre Martin, Fabrizio Melandri, Anushka Menon, Frederic Merlin, Koen Miskotte, Shai Mizrachi, István Mátis, Jos Nijland, Francisco Ocaña González, Lovro Pavletic, Jonas Plum, Tushar Purohit, Pedro Pérez Corujo, Venugopal Raskatla, Ina Rendtel, Jurgen Rendtel, Sina Rezaei, Filipp Romanov, Terrence Ross, Branislav Savic, Alex Scholten, Fengwu Sun, David Swain, Hanjie, Tamara Tchenak, Sonal Thorve, Peter van Leuteren, Hendrik Vandenbruaene, Michel Vandeputte, Ariel Westfried, Roland Winkler, Patrick Wullaert, Quanzhi Ye and Negar Yeganeh.

References
[1] Miskotte K., De Geminiden van 2017, een lastige puzzel, eRadiant 2018-1, p. 12-20
[2] Miskotte K., Geminids 2017: a tricky analysis, Meteornews 2018-3, p.114-119
[3] Miskotte K., Johannink C., Vandeputte M., Bus E.P., Geminiden: 30 jaar waarnemingen (1980-2009), eRadiant 2010-6 p. 152-186
[4] Miskotte K., Johannink C., Vandeputte M., Bus P., Geminids: 30 years of observations (1980–2009), WGN 36:9 (2011) p. 167-185