Saturday, 6 April 2013

Losing The Winter Sky

Thursday evening, at about 11, I had just got home from work so popped out to Tesco to buy some milk. And this was the sky (from Heavens Above):

The thing that sticks out is that the winter stars are getting a bit low in the west. Of course, dominating that area of the sky is the planet Jupiter - it entered Taurus on 14 May 2012, and will enter Gemini on 27 June 2013, remaining there until 7 July 2014 (see an earlier post for what Taurus and Gemini look like).

The slower-moving Saturn doesn't reach Taurus until 22 July 2029.

When I looked at the various definitions of the year, I also showed what Pisces looked like. You'll notice that along the bottom of the charts the x-axis is given in hours - these hours give the right ascension of any object in the sky.

When the First Point of Aries is due south, then the siderial time is 0 hours. You'll see to the left of the 0h is 1h - and when that line is due south, the siderial time is 1 hour.

This is siderial time - time by the stars. The First Point of Aries takes 23.9344699 hours to return to due south, and we call this the siderial day. So, one siderial hour is equivalent to 59 minutes 50 seconds.

The sky (ignoring solar system objects for the moment) takes 3 minutes 56 seconds short of a day to return to where it was. So, the sky you see at a certain point is what it will look like about half an hour earlier in a week's time and about 2 hours earlier in a month's time.

Hence the sky I saw was the same as midnight in early March (we were then on Universal Time [UT], now on Central European Time [CET]), 2 am in early February, 4 am in early January and 6 am in early December.

These evening stars are setting about 4 minutes earlier each night - but sunset is getting later, which increases the speed with which they disappear into the evening twilight.

And similarly, morning stars are rising about 4 minutes earlier each night - but sunrise is getting earlier.

We see a reverse effect from late summer to autumn.

The following charts (again from Heavens Above) show this. This is the sky at civil darkness (8.12pm CET) on 1 April:

Compare this with civil darkness (9.05pm CET) on 1 May:

Sirius, Orion and Taurus are now setting, while in the east we are startig to get the summer objects such as Vega and Hercules.

What about the morning? On 1 April, civil twilight is 6.08am CET:

The south-eastern sky is dominated by the "Summer Triangle" of Deneb in Cygnus, Vega in Lyra and Altair in Aquila. Low in the south are the zodiacal constellations of Scorpius and Sagittarius, frustratingly low - indeed parts of them are too far south to be seen even from the south coast of England. In June 1998 I was on an observing run using the Isaac Newton Telescope at the Observatorio del Roque de los Muchachos on La Palma, mainly to observe LO Pegasi. Now, on an observing run you spend most time inside, but one night I was unwell and couldn't use the telescope, but had a look out in the evening from my bedroom window and saw Scorpius in its entirety at a decent altitude and realised what we are missing in this country.

The other thing to note is how low in the east we are starting to get the autumn Square of Pegasus and its neighbour Andromeda (parts of which are always above the horizon).

I mentioned that you should expect the sky (with the exception of solar system objects) to be the same a couple of hours earlier each month. So, this sky is that at about:

  • 4am on 1 May (civil twilight 5.02am)
  • 2am on 1 June (civil twilight 4.13am)
  • midnight on 1 July (civil twilight 4.10am)
  • 10pm on 1 August (civil darkness 9.31pm)
  • 8pm on 1 September (sunset 7.51pm, civil darkness 8.25pm)
  • So, this sky is the sky about an hour before civil twilight in early May, and a couple of hours before civil twilight in early June.

    Indeed, if we look at the sky at civil twilight on 1 May, then we see not much has changed:

    And if we go on to the sky at civil twilight on 1 June:

    So, we've lost Leo, Virgo and Scorpius completely, and the autumn constellations are becoming a bit more prominent.

    So, what we see in the spring is that we lose the winter constellations quickly into the evening twilight, but sunrise getting earlier means we only slowly gain the late summer and autumn ones.

    There is a reverse effect in autumn. When I was able to actively observe (now have a flat and loads of streetlights), this was my favourite time of year. September especially - I recall one September when I had 10 clear evenings in a row.

    But there are two other factors that autumn has in its favour. The first is that the evening sky is stable. Firstly, look at the sky at civil darkness on 15 August (9.03pm CET):

    One thing to notice is this year we have Venus low in the west - it was at superior conjunction (behind the Sun) on 28 March and is now moving towards greatest eastern elongation (when it is furthest from the Sun in the sky) on 1 November.

    You might - correctly - say that 15 August isn't autumn, but this is the point in the year when the nights are starting to draw in, when you might be out in an evening and wish you'd put a jumper on, when you might have an indoor light on by the time you go to bed.

    Now move on to civil darkness on 15 September (7.53pm CET):

    Spot the difference. Well, not much really - although Venus and Saturn will be nearer each other (they will be at their closest on 20 September).

    And now civil darkness on 15 October (6.46pm CET):

    With regards to the stars, we have now lost Leo and Virgo - and indeed Saturn is now gone - and the autumn stars are a bit higher in the east. But not all that different to the civil darkness sky from two months previously.

    Note that Venus is near the bright red star Antares - they are at their closest on 16 October.

    And finally, civil darkness on 15 November (4.55pm UT):

    Compared to 3 months earlier, Leo, Virgo and Scorpius are gone. The bright orange star Arcturus is getting lower. In the east Perseus, Andromeda, Triangulum and the Square of Pegasus are higher up, and Aries and Pisces have appeared.

    This is the sky as it would be around midnight 3 months earlier - so roughly 3 hours after it got dark.

    If the evening sky is stable, the other attraction of autumn is that the morning sky is anything but! Start with the sky at civil twilight on 15 August (5.16am CET):

    By then, Jupiter will have moved into Gemini, and you might - after a night of observing the Perseid meteor shower - get your chance to wrap up the observing session by seeing Mercury.

    Starting to make their appearance are some of the winter constellations - Orion, Taurus, Gemini, while the Summer Triangle is getting low in the west.

    Now move on a month to civil twilight on 15 September (6.08am CET):

    And the sky has really moved on. Perseus, Auriga and Taurus are nearly at their best. The winter constellations - with the arrival of the bright stars Sirius and Procyon - are dominating the east and south east. Leo is making its arrival.

    And we also get Mars, down in Cancer. It is nearly at its faintest. if you look at Gemini, then there are the two brighter stars at the left end. The lower one is Pollux and the higher one Castor. Mars and Castor should be about the same brightness.

    Mars won't be at its best until 8 April 2014, when it will be about as bright as Sirius.

    If you look back at that mid-August civil twilight sky, then this will be the sky about 1/4 past 3 mid-September - roughly 3 hours before it gets light.

    We now move another month on to civil twilight on 15 October (6.56am CET):

    The winter constellations and Jupiter dominate the southern sky. The familiar autumn constellations like Pegasus and Pisces are starting to set. Out of the Summer Triangle, only Deneb is visible, low down in the north/north-west.

    Having set in the evening, Arcturus has now risen again and is low in the east. Leo is prominent in the east.

    That mid-September civil twilight sky is now the sky at about 1/4 past 4 (about 2 3/4 hours before it gets light) and the mid-August civil twilight sky is now the sky at about 1/4 past 1 (about 5 3/4 hours before it gets light).

    And finally, civil twilight on 15 November (6.45am UT):

    Even though it is still autumn, the main winter constellations are getting low. In the east, the sky is domiated with the spring constellations, with Vega - like Arcturus - have risen after setting earlier in the night.

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