A video of Sirius (the brightest star in the night sky) twinkling on the horizon, captured by Steve Brown @sjb_astro. Many people think they’re seeing a low altitude aircraft or UFO when they witness this.
This phenomenon occurs (to a lesser extent) with any bright star low on the horizon due to the vast amount of atmosphere you’re seeing it through. As stars gain elevation, and less atmosphere is between us and them, they shine more steadily, and views are hugely improved.
The difference in the amount of atmosphere you look through with elevation is very striking (as demonstrated in the sketch below). Particularly for faint deep sky objects like galaxies, high elevations makes a dramatic difference to the quality of visual or photographic images you’ll collect.
How much atmosphere you look through with observing angle. At the zenith (overhead) you look through over 2/3 less atmosphere than at the horizon.
A stunning sunset captured over the Isle of Rum
In the north of Scotland we’re about four days away from losing all astronomical twilight and entering a period of sustained ‘nautical twilight’.
During this time the centre of the Sun’s disk never dips more than 12 degrees below the horizon, rendering our clear night skies a dark azure blue, with only the Moon, planets and brightest stars visible after midnight.
This will continue until mid July when astronomical twilight finally reappears.
Contrast this with London, where astronomical twilight continues right through mid summer, producing much darker night skies, but arguably less beautiful and prolonged sunsets.
We’re leaving astronomical twilight behind for several months here in the north of Scotland
Significantly darker night skies persist through mid summer in southern England. Here’s the contrasting data for London.
Mercury is only 40% larger than the moon but very difficult to spot
The planet Mercury can be very tricky to observe. It’s close proximity to the sun means we generally only have brief opportunities to observe it low on the horizon either before or after sunset.
Right now Mercury is approaching maximum eastern elongation (on March 15th to be precise) meaning the planet is up for longer after the sun sets. The window is still pretty brief with only about 45 mins of useful time to work with after sunset.
Your best chance is to pick a clear evening and head out somewhere with a good unobstructed view to the West. You don’t need dark skies as the Sun will still be producing a lot of light between 6 and 8pm.
At the moment, and at Highland latitudes, the action starts about 6.30pm just after the Sun sets. Wait a while then scan the western horizon and you should see Venus first, which will appear brighter. Use this as a guide for finding Mercury which will sit slightly above it over the next few days.
6.20pm 14th March 2018, 57 degrees north
The longer you wait after the sunset the easier Mercury will be to see due to darkening skies but also harder due to it moving lower and lower towards the horizon, adding more atmospheric distortion to your views.
If you do see it take a note of its crescent phase. We almost always see Mercury as a crescent because it would be too close to the sun to see it in a full or new aspect. One exception to this is during a solar transit when Mercury crosses directly across the disc of the sun. The next opportunity to witness this will be 11th November 2019, which gives you plenty of time to prepare a solar filter for safe observing of the solar disc. Happy planet hunting meanwhile.
Transits reveal the true scale of the Sun
One of the most energetic meteor showers of the year is fast approaching with activity predicted between Dec 4th – Dec 16th. With the best possible observing conditions the Geminids can produce displays of up to 120 meteors per hour, although you’ll likely see rates much lower than this in reality.
Occasionally and unpredictably, meteor showers can erupt into storms. One of the most famous ‘storms’ happened in 1833 when the Leonids produced over 100,000 meteors per hour! Who knows what this December will bring.
Observing Meteor Showers
You don’t need any special equipment to view a meteor shower, in fact binoculars or telescopes will just narrow your field of view. Grab a deck chair or a warm blanket, prepare a hot drink, wrap up warm and lay out under the darkest conditions you can find. It’s an excellent activity to do alone or if you have children they’ll love an excuse to get outside for some after dark play.
Put away any lights or bright mobile phones and simply look up and wait. Remember it takes up to 30 minutes for your eyes to fully dark adapt and any exposure to bright lights will start the process all over again. If you need a light red touches are best for preserving you night vision.
Good luck and clear skies!