Martin Dawson joined the York Astronomical Society in
1973 shortly after the society was formed and has been a member ever since. He
kept an occasional diary of events from that time. Some of the entries bear a
similarity to current happenings at the Society: meetings, talks and working
parties at the observatory, then at Acaster Airfield. An example entry:
7 Jan 1977 – ‘YAS Member Mrs. Gibson presented her talk on her trip to West Africa to see the 1976 October eclipse. 0.90p made in raffle (1.75) Planisphere as prize.’
The Society’s first crescent moon watch (Saturday 6 April 2019)
York Astronomical Society’s first crescent moon watch at Beetle Bank Farm near York was a big success – just a pity the weather didn’t oblige by giving us a clear view! The event was a Society initiative with the local Islamic community for whom the crescent moon is of religious importance. It was a trial run for what is hoped will become a regular event.
The following article often refers to angles in the sky.
The diagram above is a rough guide. Hand at arm’s length (courtesy of timeanddate.com).
British Summer Time and the meaning of ‘dark’
As the clocks go forwards on Sunday March 31st, we are now
in British Summer Time (BST). All timings given in this article will therefore
be in BST. For astronomers, it means of course that we have to wait one hour
longer before it gets dark.
What do we mean by ‘dark’? There are four ways we might
define it!
Lighting up time
This is when – according to the Road Vehicles Lighting Regulations 1989 – road vehicles must put their headlights on (unless in a well lit 30mph area, in which case only sidelights are required). And the regulations state that this time is 30 minutes after sunset.
Civil Twilight
This is when the sun drops to 6° below the horizon. Bright stars and the five naked eye planets can be seen and clusters can be seen in binoculars, but no deep sky objects are visible, even with a telescope.
Nautical Twilight
This is when the sun drops to 12° below the horizon. Mariners consider that this is when it gets dark. This isn’t bad for astronomers. There’s a glow north of where the sun set but most deep sky objects are observable.
Astronomical Twilight
This is when the sun drops to 18° below the horizon. Astronomers deem it only to be completely dark if the sun is this far down. The faintest deep sky objects can now be seen.
The table below gives these timings and sunsets for York for
four dates in April. Remember, all are BST.
[table id=1 /]
Notice how, at the beginning of the month, astronomical twilight is 2hrs 8mins after
sunset, but by month end it is 2hrs 50mins after sunset. This is because, as
the suns moves northwards on the celestial sphere, it sets at an ever
decreasing angle to the horizon so takes longer to dip 18° below it. In fact,
by 7th May, there is no astronomical twilight, complete darkness returning for
a brief time centred on 0100 on 1st August.
So what is all this telling us? It’s saying that April is
the last month that the sky gets really dark, so if you want to see deep sky
objects, get out there before it’s too late – or wait until August. Also, be
aware of the Moon, which will produce background sky glow for much of the
second half of the month.
What to See in April
The Moon
Best seen as a waxing crescent through first quarter to
gibbous from Mon 8th to Tues 16th.
Planets
Mercury, Venus, Jupiter, Saturn, Uranus and Neptune are all
either impossible to see because they are too near the Sun, or very poorly
placed for observing this month.
Diminutive Mars (now at mag. 1.5 and only 4 arc secs in
diameter) can be seen due west and 9° above the Moon on the 8th. Only with
powerful telescopes and under perfect seeing is there any chance to see any
surface markings. We can wave goodbye to Mars now, until its return to the
morning sky in August 2020. It reaches a fairly favourable opposition on 13th
October 2020.
An Asteroid Challenge
The asteroid Pallas
reaches opposition on the 4th April, and on the 10th it passes very close to
the star Muphrid (Eta Boötes) giving us a rare opportunity to spot it easily.
Pallas is at magnitude 7.9 and Muphrid is at magnitude 2.7. It’s easy to hop
from Arcturus (Alpha Boötes at magnitude 0.15) to Muphrid (about 5° to the WNW
of Arcturus) where you will see Pallas just ¼° away from Muphrid in the direction of
Arcturus. In fact, due to its large inclination (35°) to the plane of the
ecliptic, Pallas is travelling northwards and if you look on the 9th, 10th and
11th, you will see it changing its position.
The first sky diagram below shows where Boötes and its
brightest star Arcturus are at 2130; the second is a finder chart approximately
5° wide. A small telescope or even binoculars will show all the stars in this
chart.
Stellarium screenshot showing the constellation BoötesStellarium screenshot showing the position of Pallas.
Deep Sky Objects
Last chance before the light nights, and best seen before the
first quarter Moon on the 12th or after the 20th. If you can wait until after
nautical twilight, then do so. And for the first 10 days in April, astronomical
twilight at about 10pm is still within social hours.
There follows a shortlist of objects – all shown on the general
wide-field sky chart below – that are roughly in the south at 2130 hrs between
the 1st and the 10th or after 11pm from the 20th.
Deep sky objects in April
M44 – Beehive Cluster (aka Praesepe)
An open cluster in Cancer of about 1000 stars with a
combined magnitude of 3.7. Easily visible to the naked eye on a clear, dark
night. Best seen with binoculars or any eyepiece-telescope combination that
delivers a field diameter of 1½°. That’s three times the apparent diameter of
the Moon.
The Leo and Virgo Galaxy Clusters
The detailed sky chart below shows the location of these
famous clusters of galaxies. April is the best time to see these before
midnight, and your last chance until February next year.
A famous trio of galaxies of about 9th or 10th magnitude all
visible in the same low power field. It’s possible to see them with a 3″
refractor but larger apertures will make spotting them much easier.
These two galaxies are the brightest close pair within the
massive Virgo cluster. It’s worth just slowly panning across this area of sky
with a wide field eyepiece to see how many galaxies you can spot.
Space news
Well, he did it! Elon Musk, the triple-nationality (South
Africa, Canada, United States) entrepreneur CEO of SpaceX and possessed of a
personal fortune of 23 billion dollars, has succeeded in launching his unmanned
Crew Dragon spacecraft to dock with the ISS and return it safely to earth.
After this “Demo-1” demonstration mission, SpaceX plans to conduct an
in-flight test of Crew Dragon’s emergency-escape system; if all goes well with
that un-crewed launch, Crew Dragon could carry two NASA astronauts to the ISS
on the Demo-2 demonstration flight as early as July.
York Astro member Martin Dawson, has sent us a copy of the Society’s newsletter from 1974, two years after the Society was formed. Although the newsletter was printed on old technology with hand drawn illustrations, the topics covered would be familiar to members now; reports on recent talks, what’s to be observed that month and progress with the observatory. Back then, meetings were held at the Railway Institute and planning permission for the observatory had been obtained, plus the Society has acquired a 12.5″ reflecting telescope – wonder what happened to that.
Inclement weather threatened us at the start of the day, but
it eventually turned sunny. There was a good turnout of volunteers and a number
of tasks completed. After setting the world to rights we started work. We were
refuelled at lunchtime by some great bacon and sausage baps from Angela.
Talk by David Cook at Priory Street, 1 March 2019.
There is more to sundials than things plonked in a garden on screwed to a church wall and you would be surprised how accurate they can be.
David presented an interesting talk for the Society at the Priory Street Centre on Friday 1 March. He started out by saying that sundials are ‘rubbish’ and then went on tho show that a lot of them aren’t, demonstrating a range of different sundials from his own collection. Continue reading →
A monthly look at astronomical events in the sky and on Earth
March 2019
The following article often refers to angles in the sky. The diagram above is a rough guide. Hold your hand at arm’s length. (Courtesy of timeanddate.com)
Fri/Sat/Sun 1st/2nd/3rd
A rare opportunity to catch a naked eye glimpse of elusive Mercury in the evening sky after sunset. Find a place where you’ve got a good view of the western horizon, note where the sun sets (at about 1740), wait until 1815 then look at a point 10° directly above where the sun set and there will be Mercury. Binoculars will help you to spot it initially but it’s easily visible without them. A small telescope will show it as a 36% illuminated orange coloured crescent. It’s visible until after 1845 as it slowly approaches the horizon.
BEWARE! Don’t try using binoculars to scan the sky for Mercury before the sun has completely set. You could blind yourself!
Sat 2nd
If you’ re awake by 0615, look low in the SE to see Venus and the crescent Moon together just 5° above the horizon. A nice sight.
Fri 8th
A wafer thin crescent Moon appears in the WSW after sunset (1754). It’s only 4% illuminated so will be a challenge to spot but it’s worth a try. By 1830 it should be easier to see, and Mercury may still be spotted at 5° altitude due west.
Mon 11th
The waxing crescent moon passes near to Mars. Spot the red planet 3° above and to the right of the Moon any time from 1900 to 2100 in the SW to W.
Thu 14th
Moon at first quarter. This is a great time to observe it, as the terminator is facing us and throwing lunar features into sharp relief.
Objects of the Month
There follows a shortlist of objects that are roughly in the south this month at 2030 hrs – a convenient time for family viewing.
Click on image to enlarge
The image above shows the sky at about this time, looking south (from Stellarium).
Castor and Pollux
The “heavenly twins” and the two brightest stars in the constellation of Gemini.
A 2 or 3-inch telescope will show Castor as a double star (separation 4 arc seconds) but in fact each of those stars is itself a double, discovered spectroscopically as they are too close to be separated optically. Each pair comprises a hot, bright A-class main sequence star orbited by a cool, faint M-type red dwarf. What’s more, this pair of doubles is orbited by another very faint double, making Castor an amazing 6-star system. Its combined magnitude is 1.58.
While your telescope is pointing at Gemini, take a moment to look at Pollux. Its orange colour is clearly very different from that of Castor. Pollux is a giant K-type star nine times the diameter of the Sun but only twice its mass. At 34 light years distant, it’s the nearest giant star to us. It does have a planet at least twice the size of Earth orbiting it called Thestias. Unfortunately, at only 1.6 astronomical units (a.u.) from the star when the habitable zone round Pollux is between 6 and 12 a.u., life on Thestias could be a tad challenging!
The Winter Triangle (Procyon, Sirius and Betelgeuse)
These three interesting stars form a near-equilateral triangle. See star map above.
Sirius
Hubble image (NASA)
SiriusThe brightest star in the sky. Is actually a binary. Sirius A is a main sequence A0 star; Sirius B is a white dwarf hundreds of times fainter than Sirius A. Although their angular separation is currently 10 arc seconds, the difference in brightness (magnitudes -1.44 and 8.5) makes spotting Sirius B a challenge for even large telescopes. The Hubble image on the left illustrates this.
Procyon
Another binary star. Procyon A is an F5 star and thus cooler and yellower than Sirius. Procyon B is another white dwarf. Their magnitude difference and closer separation (4.3″) makes spotting the fainter component even more difficult than for Sirius but Giuseppe Donatiello has done it with a home made 127mm f/9 ED refractor!
Betelgeuse
Image courtesy of NASA
Now Betelgeuse is something quite extraordinary. It’s an M1 red supergiant and one of only a handful of stars that are large and near enough for specialist telescopes to see their surface as a disc. This is a NASA image. Put it where the sun is and it would extend nearly to the orbit of Jupiter. It’s a semiregular variable whose magnitude changes by 1.3; its surface temperature is only 3600K but it’s luminosity is 90,000 to 150,000 times that of the sun.
And finally, two Open Clusters, M41 and M47
Click image to enlarge
These often neglected clusters are fine targets in modest telescopes and even binoculars. M41 is 5° below Sirius, and M47 is 12° east and 2° north of Sirius. They’re both at magnitude 4.5 and about the apparent diameter of the full moon. Note that M41 contains a mixture of red giants and white dwarfs but M47 comprises mainly young blue stars and only a few red giants.
M41
Image courtesy of NASA
M47
Image courtesy of NASA
Stop press – space news
As this item is being uploaded, we have news of the successful launch of Israel’s Beresheet lunar lander. This privately funded mission by Israel’s SpaceIL company lifted off using a SpaceX Falcon 9 rocket. The name Beresheet comes from the book of Genesis and means “In the beginning”. Clearly, Israel intends this to be the beginning of their efforts in space. For the latest news, see this Sky & Telescope article and this Beresheet fact sheet.
People in York (well, those prepared to be up through the night) were rewarded with a spectacular view of the lunar eclipse on Monday morning. I feel sorry for some I know who got up through the night and see clouds, only to go back to bed to miss the spectacle when they cleared. Continue reading →
As 2019 dawns we are already witnessing some amazing astronomical events – the New Horizons encounter with Ultima Thule and the China mission Chang’e 4.
2018 ended for YAS with our observatory relocated to Beetle Bank Farm and work under way to recommission.
The remaining Portakabin unit has been dismantled and suitable materials recovered for recycling or appropriate disposal. The remaining rotted wood has been burnt. Here you can see Martin Whipp in nostalgic mood watching the burning embers – Martin was there at the beginning and at the end. Continue reading →
