Clear skies?

You can’t con­trol the weather, but by be­ing aware of what’s oc­cur­ring in the at­mos­phere you can pre­pare for the night ahead

Sky at Night Magazine - - EXPLAINER -

The sky of­ten looks ideal for stargaz­ing, but looks can be de­ceiv­ing. Am­a­teur ob­servers de­scribe view­ing con­di­tions us­ing terms such as ‘trans­parency’ and ‘see­ing’. Trans­parency is straight­for­ward: ob­serv­ing a sky with poor trans­parency is like try­ing to look through a dirty win­dow, but in this case the dirt is high, in thin clouds, at­mo­spheric dust or mois­ture, or even air­craft con­trails.

See­ing de­scribes how steady or tur­bu­lent the at­mos­phere is and it can be es­ti­mated by ob­serv­ing brighter stars with the naked eye. When the see­ing is poor, stars ap­pear to twin­kle more. Weather fore­casts for astronomer­s can be help­ful for an­tic­i­pat­ing ob­serv­ing con­di­tions (try en. sat24.com or www. clearout­side.com) – if good con­di­tions co­in­cide with a dark new Moon pe­riod, the view should be out­stand­ing.

Am­bi­ent tem­per­a­ture also plays a role, which is why tele­scopes need time to ac­cli­ma­tise when you take them out­side – al­low­ing an hour of cool­ing be­fore you start ob­serv­ing re­ally helps. Even so, a tar­get that looks poor one night may be spec­tac­u­lar the next. A worth­while ex­per­i­ment is to ob­serve a bright lu­nar limb (the vis­i­ble ‘edge’ of the Moon) at high mag­ni­fi­ca­tion and note how the view ap­pears to wob­ble. This is the ef­fect of the see­ing. Lin­ger­ing on the view a few min­utes will re­veal patches of im­proved see­ing, let­ting more de­tail shine through. Ex­pe­ri­enced ob­servers will take their time over each tar­get and keep com­ing back to favourite ob­jects over the years.

Fo­cus on the bright limb of the Moon to gauge how good or bad the ‘see­ing’ is

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