How To...

…Record the Gem­i­nid me­teor shower.

Sky at Night Magazine - - CONTENTS - With Paul Abel

De­cem­ber’s cold and frosty nights mark the ar­rival of the year’s rich­est me­teor show­ers: the Geminids. The shower runs from 8-17 De­cem­ber with peak ac­tiv­ity oc­cur­ring on the morn­ing of 14 De­cem­ber. This year the Moon sets be­fore 21:30 UT so the dis­play won’t be drowned in lu­nar glare. Here, we’re go­ing to look at why these ob­ser­va­tions are use­ful to re­searchers and how best to ob­serve the Geminids and record them.

All me­teor show­ers are pro­duced by ei­ther a comet or an as­ter­oid that we call the parental body. The Geminids are caused by 3200 Phaethon, an Apollo-type as­ter­oid that or­bits the Sun every 1.4 years. 3200 Phaethon has a rather ec­cen­tric or­bit; the fur­thest it ever gets from the Sun is a dis­tance of 2.4 AU (aphe­lion) but its clos­est dis­tance (per­i­he­lion) is just 0.1AU – closer to the Sun than the planet Mer­cury. 3200 Phaethon has the dis­tinc­tion of be­ing the only as­ter­oid in the So­lar Sys­tem to get this close to the Sun!

While it’s in the in­ner So­lar Sys­tem, heat from the Sun causes par­ti­cles to es­cape from the sur­face of Phaethon and fly away into space. Even­tu­ally, these par­ti­cles col­lide with Earth; they burn up as they hit the at­mos­phere, caus­ing a glow­ing trail, or what we call a me­teor or ‘shoot­ing star’.

Since Earth is col­lid­ing with a cloud of par­ti­cles, the me­te­ors ap­pear to us to come from one di­rec­tion in the sky; this is called

the ra­di­ant and the lo­ca­tion of the ra­di­ant gives the name of the shower. The ra­di­ant for me­te­ors in the con­stel­la­tion of Gemini gives the shower its name: the Geminids.

Me­teor show­ers are a good op­por­tu­nity for as­tronomers to study their parental bod­ies, so am­a­teur ob­ser­va­tions are

par­tic­u­larly wel­comed by or­gan­i­sa­tions such as the Bri­tish As­tro­nom­i­cal As­so­ci­a­tion (BAA). If, for ex­am­ple, anal­y­sis of the ob­ser­va­tions re­veals that the ra­di­ant has shifted, this could in­di­cate or­bital changes. We can also in­fer how ac­tive the parental ob­ject has been dur­ing its pas­sage into the in­ner So­lar Sys­tem.

Make it a group ac­tiv­ity

Use­ful ob­ser­va­tions of me­te­ors re­quire an or­gan­ised me­teor watch. You can look for me­te­ors on your own, but it is com­mon to ob­serve in groups with one mem­ber act­ing as a recorder and fill­ing in the form as the var­i­ous ob­servers call out their sight­ings. The re­port form used by the BAA is avail­able elec­tron­i­cally, so you can print it out and use it dur­ing your ses­sion.

Al­though the Geminids have a Zenithal Hourly Rate (ZHR) of over 100 me­te­ors an hour, this num­ber can be mis­lead­ing since it as­sumes op­ti­mum view­ing con­di­tions: a ra­di­ant that’s high in the sky and no light pol­lu­tion or clouds. In re­al­ity you won’t see as many as this. Geminids tend to be swift and usu­ally white in colour – the streak they leave be­hind is called a me­teor train and it may per­sist for a few sec­onds.

You’ll need to record the names of all ob­servers, your lo­ca­tion and con­tact de­tails along with the start and end times (in UT) of your ses­sion. You also need to make a note of the stel­lar lim­it­ing mag­ni­tude of your site by lo­cat­ing and iden­ti­fy­ing the faintest star you can see with the naked eye.

As the watch gets un­der­way, you’ll need to record the times the me­te­ors were ob­served along with their es­ti­mated mag­ni­tude. Re­mem­ber, not all of the me­te­ors ob­served will be Geminids! A true Gem­i­nid will have ei­ther a short train close to the ra­di­ant or a longer one fur­ther away; it must also be mov­ing away from the ra­di­ant. If your me­teor is not obey­ing these rules it is what’s known as a ‘spo­radic’ and not part of the shower – you should still record it but la­bel it ‘spo­radic’. You also need to in­clude any con­stel­la­tions that the me­teor passes and how long it takes to fade.

Once in­doors, you should type your ob­ser­va­tions into the re­port form and save it on your com­puter so that you have a good per­ma­nent record. And don’t just let the data you’ve col­lected sit around. Send your ob­ser­va­tions to the Me­teor Sec­tion of the BAA – the sci­en­tists there will make good use of the data!

The fur­ther away a me­teor ap­pears from the ra­di­ant of the shower, the longer its trail will be

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