Why this part of Andalucia?
So far as the reasons for choosing Andalucia as a site for astronomy, they were a compromise between a good astronomical location, a choice of flights from the UK and having basic facilities, such as water and electricity supplies.
The main factors that make a good astronomical location are:
There are some other benfits from being close to the equator, too. The first
is that as the seasons change, there is more variation in what's visible. This
means that through the year, there are more things to see, the closer you get
to the equator.
The second aspect is that as you get closer to the equator, the difference between the length of the night from summer to winter is reduced. While this means that winter nights are longer further north, it also means in summer they are shorter in northern latitudes. In britain for instance, during the month or two on each side of the summer solstice (late June) the sky never really gets dark as the time between sunset and sunrise is so short.
When you take this into account, it adds an extra factor into the search for a good astronomy site: getting as far south in Europe as possible. I had decided to limit my search to Europe simply because anywhere further would involve much longer travelling times and would be hard to drive to, for extended stays.
Inland areas are more concerned with crop-growing and farming than with leisure activites and therefore do not publicise sunshine records. I did find out that the province of Almeria contains Europe's only official desert. This sounded promising, as did the fact that there is a joint German-Spanish observatory in the region. Which is 50km (30 miles) from my house and visible on clear days.
The effect of light pollution is to make the sky over the lit-up town appear
brighter. This can be seen from a distance as a "dome" of light on the horizon,
although from inside the town itself, peoples' eyes never adapt to true darkness
so they will not realise that the sky is not as dark as it should be. The
only affect they may notice is the lack of stars, if they ever care to look up.
From an astronomical point of view, the increased brightness of the sky means that a lot of the dim objects are washed out or even invisible. It's similar to listening to music with a pneumatic drill digging up the road just outside your window; you can still hear the loud bits, but all the nuances disappear.
What I wanted to know was which areas of Spain had the worst light pollution and which areas are still relatively free. A lot of work has been done by groups like this to map the areas in europe that are affected by light pollution. I took their map of light polluted areas and overlaid it with a map of the major towns and cities in Spain, (the map on the left). As you can see, there is a good match, once you take into account the different perspectives of the two maps, between major towns and major light pollution.
The colour coding shows sky darkness, ranging from the darkest areas in grey through blue, green, yellow and with the worst polluted areas as white.
Finding the right locationI already knew from my research into sunshine/clouds that the south-eastern corner of Spain looked quite promising. The light pollution plots showed that there was a light-free pocket inland from the city of Almeria, stretching east.
See the zoomed in map on the right.
Based on this information, I started looking in the area centred on a small town called Lubrin.
This made me reconsider my approach and I realised that the reason this area looked so good on paper meant that there were practical drawbacks to living in a sparsely populated area.
I changed my search area to one of the other pockets of blue on the map.
This time looking around the area to the north of Baza, where the observatory
at La Sagra is located. I struck lucky by
hooking up with an estate agent who "got" my requirements. Apart from
showing me the house I eventually bought, he also showed me a place
quite close to the Calar Alto observatory between Baza and Almeria. While this was a
great location, it did suffer from the drawbacks of being literally in the
middle of nowhere. I was concerned about being this cut off - there were no neighbours
or even a mobile phone signal, let alone a landline. This could be a problem if there was
ever an emergency.
The location I eventually chose is between these two observatories.
While the light pollution map is a good indication of large-scale effects it is not designed, nor able, to tell you how good or bad a particular point will be. For that there is no substitute for going and checking a location yourself.
In practice a person with average eyesight can see stars as dim as magnitude 6, provided the sky is dark enough that stars this dim aren't blotted out by light pollution. Astronomers frequently describe the darkness of the night sky in terms of the dimmest stars they can see, using just their eyes. For example in the middle of a town, excess light means that your eyes never get fully dark-adapted and therefore never reach their greatest sensitivity. For that reason, and the consequent lack of contrast between the bright sky and the dim stars, the dimmest stars that a person can see may only be magnitude 2 or 3. That person would say therefore, that the sky has a limiting magnitude of 2 or 3.
Because these measurements are dependent on individuals, they can be
somewhat unreliable. A person with good eyesight can see dimmer stars that
someone who's eyes are not so good. Another factor that comes into play is
how experienced an observer is: for example a veteran observer can use
"averted vision" by not looking directly at a star, to improve their eyes'
Fortunately it is now possible to get an instrument that removes the subjective element from sky quality measurements and can prevent arguments about just how good or bad a particular location is.
Using one of these meters, I measured the sky brightness at Cuevas del Negro
to have a limiting magnitude of 6.3.
To put this into perspective, at my house in Marlow, the limiting magnitude
on an average night is 3.5 - on a good night it sometimes gets down to 4.
Here are two photos taken with a fisheye lens that shows almost all the
sky. The one on the left is a 60 second exposure taken on a dark night
from a field a couple of miles from my house in Marlow. The photo on the right is the same
exposure time taken at Cuevas del Negro.
In numerical terms, the difference between Magnitude 3.5 and 6.3 (i.e. 2.8 Magnitudes) is that Magnitude 6.3 is about 13 times darker, since the scale is logarithmic.
|Summer night from a dark location at Chisbridge, near Marlow, about 1:00 a.m.
Note faint view of the
milky way running across the image
|Winter view from my house in Cuevas del Negro, north to the top.
Orion visible in lower quarter of image