Tue, 01 Dec 2015 - Getting Ready for the Geminids (part 1) : Basics
Although the peak of the Geminid meteor shower is still nearly 2 weeks away, it is worthwhile making plans now so that you can get your best view of this year's Geminid meteors.
Which will be the best night(s)?
Geminid activity can be seen from around Dec 7 through to around Dec 16. However, as this IMO Activity Curve from 2012 shows, most of the activity occurs however over three or four nights near the peak.
Although predictions quote a time during the afternoon of Dec 14th for the peak of the 2015 Geminids, the peak is actually quite broad and Geminid rates during the nights of both Dec 13-14 (Sun-Mon) and Dec 14-15 (Mon-Tue) will be close to the peak value.
Rates should also be quite good during the night of Dec 12-13 (Sat-Sun) - which is good news for anyone unable to observe far into the nights of Dec 13-14 or Dec 14-15 due to next day work commitments.
Great news for 2015 is that New Moon occurs on Dec 11th, so moonlight will not pose a problem.
When to observe each night
Geminid meteors appear to radiate outwards from a location near to the bright star Castor (Alpha Geminorum). The higher that this is in the sky, the more Geminids will be seen.
The accompanying graph shows how the altitude of this radiant area changes as the night progresses.
With Gemini being rather low in the sky at the start of the evening, the rate at which Geminids will be seen during the early evening will be fairly low. However, Gemini climbs higher into the sky as the evening progresses and, as a result, observed Geminid rates rise.
The Geminid radiant is highest in the sky shortly before 2am (this contrasts with showers such as the Perseids, Orionids and Leonids for which you have to stay out until near the end of the night to see the best observed rates).
Hence the best Geminid rates will be seen in the early hours of the morning.
However, good Geminid rates should be visible from mid evening onwards, so you may wish to start observing at 9pm or 10pm and then watch observed Geminid rates rise as the radiant approaches its high point.
If work commitments prevent you from observing beyond midnight on Dec 13-14 or 14-15, you might still want to consider observing from 9pm until around midnight.
How many Geminids will you see?
In recent years, the peak Geminid ZHR has typically been in the 100-110 range. However, despite the impression given by many poorly written on-line news stories (some of which inflate the figure to 120), the observed rates that you see will not be as high as this ZHR value. Be wary of websites and Facebook/Twitter posts that promise high rates such as “up to 100” (and only mention in the “small print” that most people will see considerably less than 100 per hour).
Two main factors will impact the number of Geminids that you will see.
The first is how much of the sky you can see. If part of your field of view is blocked by buildings, trees, etc, these will hide any meteors that appear behind them. Therefore, choose an observing site that has a clear view of the sky.
The second factor, and most critical, is the darkness of the sky background. Although the Geminids are described as being “rich in bright meteors”, for every bright Geminid that you will see, there will also be quite a few fainter ones. If the sky is lit up by nearby streetlighting , the fainter Geminids will be “lost” against this bright sky background. Therefore, try to find as dark an observing site as possible and do make sure that you are in the shade from any nearby lighting.
From a reasonably dark observing site with a good view of the sky, you can probably expect to see 40-50 meteors per hour when the Geminid radiant is high in the sky during Dec 13-14 and 14-15. This figure is somewhat lower than the published ZHR value, but is nevertheless an impressive number of meteors.
There is a third factor that can affect the number of Geminids that you see. December nights with clear skies tend to be rather cold. Make sure that you are well wrapped up against the cold, paying particularly attention to your feet, knees, hands and head. You want your attention to be focussed on seeing meteors and not be distracted by the cold!
Watch out for part 2 of this guide
This will be published in the next few days and will contain more useful tips about observing the 2015 Geminids.
Added by: Tracie Heywood