ASTRONOMICAL SOCIETY OF BOTSWANA
Why Astronomy in Botswana ?
Map
Map of World Cloud Cover Averaged from 2002 to 2015 (NASA)
The rainy season in Botswana begins in October and finishes in April. The 5 months in between have little to no rain resulting in mostly clear nights – ideal for astronomical observation. While this year, 2021, has recorded higher than average rainfall in many parts of the country, some years record no rain from April through October, giving 7 months of potential clear viewing conditions.
The months of June, July and August rarely experience any rainfall and allow for the long-term planning of astronomical viewing and imaging events, trips and educational opportunities.
Original chart compiled by ASB with data extracted from various sources
The highest elevation in Botswana is 1489m on top of Otse Hill, Southwest of Gaborone. However, most of the country, other than the far Eastern and far Southwestern extremes, is above 1000m, or about 3280 feet, above sea level.
The Western and South central parts of the country, where elevations are high, corresponds to low population densities. This results in low ambient light conditions making stargazing conditions ideal.
It can be seen from the map above that most of Botswana has a population density of less than 1 person per square kilometre.
Few countries in the world have such low population densities, making dark skies the rule, not the exception, in Botswana.
Low population density not only results in less ambient light, it also results in lower levels of other frequencies of electromagnetic radiation. This is why the Southwestern part of the country has been chosen as the location for one of the dishes for the SKA (square kilometre array) radio telescope project which should be completed by the end of the decade. It will be the largest telescope (non-optical) in the world.
We are especially well situated on the planet, here in Botswana, to be able to see more stars than from almost anywhere else on Earth. This is true because on the longest night of the year (the winter solstice – this year happening at 23:43 Botswana time on June 20th), the galactic core (Milky Way Center) is situated almost directly overhead (See “Zenith” in the center of the above image – 4ᵒ away from the galactic center) in the constellation Sagittarius. This overhead position of the Galactic Center allows us to look back towards the center of the galaxy, where the densest array of stars and deep space objects can be found. This overhead position also allows us to look through the thinnest part of the atmosphere for the most hours, and, the end of June is one of the times of year when we, in Botswana, have the clearest skies. All in all it provides for a huge advantage to anyone wishing to view, or photograph, the Milky Way – including many of the brightest stars and most beautiful deep sky objects that can be seen from anywhere on the planet.
Galaxies
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.[1][2] The word galaxy is derived from the Greek galaxias (γαλαξίας), literally “milky”, a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million (108) stars to giants with one hundred trillion (1014) stars,[3] each orbiting its galaxy’s center of mass.
Galaxies are categorized according to their visual morphology as elliptical,[4] spiral, or irregular.[5] [There is also a further sub-division of spiral galaxies with a ‘bar’ of stars extending each side of their centre – ‘barred spirals’. The Milky Way is thought to be one of these]. Many galaxies are thought to have supermassive black holes at their centres (see below).
The latest estimate is that there are more than 2×1012 (two trillion!) galaxies in the observable Universe.
Our ‘Milky Way’ Galaxy
The Milky Way is the galaxy that contains our Solar System, with the name describing the galaxy’s appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. Wikipedia
Radius: 52,850 light years
Stars: 250 billion ± 150 billion
Sun’s Galactic rotation period: 240 Myr
Sun’s distance to Galactic Center: 26.4 ± 1.0 kly (8.09 ± 0.31 kpc)
Escape velocity at Sun’s position: 550 km/s
Spiral pattern rotation period: 220–360 Myr
The European Space Agency have produced a neat video of the Milky Way as part of their Gaia Mission – https://www.youtube.com/watch?v=G5AdrupH788 – this is in the form of a journey from the centre and includes the main features of our Galaxy (traditionally given a capital ‘G’ when talking about our Galaxy as opposed to any other).
Here’s an ESA schematic of the main structures of the Milky Way (and other spiral galaxies):
The Galactic Centre
The Galactic centre is the point about which our Galaxy is rotating. It is located roughly 24,000 light years from the Solar System in the direction of the constellation Sagittarius, but cannot be seen in optical light due to heavy obscuration by interstellar dust grains along the line of sight. It is, however, observable at wavelengths that are not as affected by dust, in particular at infrared, radio and X-ray wavelengths.
Observations have revealed a complex radio source located very close to the Galactic centre. In particular, the compact radio and X-ray source Sagittarius A (SgrA*) has long been thought to be the location of a supermassive black hole at the centre of our Galaxy.
This idea has gained strength through recent infrared observations which were used to plot the orbits of stars located within light hours of the Galactic centre. It was found that these stars have very tight and fast Kelperian orbits, around an object of about 3 million solar masses located at the position of SgrA*. The orbital characteristics of these stars indicate that this mass cannot be due to compact clusters of neutron stars, stellar size black holes or one of the many other suggestions that have been put forward over the years. The most likely explanation is that a supermassive black hole, similar to those that have been observed in the centres of other galaxies, also lies at the centre of our own.
An animation showing the orbits of stars observed in infrared radiation near the Galactic centre over several years is at https://www.youtube.com/watch?v=Eysecnh7yqc Such observations have allowed us to determine the mass of the central black hole from the simple application of ‘Kepler’s Laws’.
In 1958 the International Astronomical Union (IAU) decided to adopt the position of Sagittarius A as the true zero co-ordinate point for the system of galactic latitude and longitude. In the equatorial coordinate system the location is: RA 17h 45m 40.04s, Dec −29° 00′ 28.1″ (J2000 epoch) (see below for the implications of this position).
Baade’s Window
This is an area of the sky with relatively low amounts of interstellar “dust” along the line of sight from the Earth (i.e. there is not as much extinction of light as there would be otherwise). This area is considered an observational “window” therefore. It is named for astronomer Walter Baade who first recognized its significance. This area corresponds to one of the brightest visible patches of the Milky Way. It is centered at Galactic longitude l=1.02 degree and Galactic latitude b=-3.92 degrees (i.e. not quite towards the Galactic centre itself).
Baade’s Window is frequently used to study distant central bulge stars in visible and near-visible wavelengths of light. Important information on the internal geometry of the Milky Way is still being refined by measurements made through this “window”. It is in the direction of the constellation of Sagittarius.[3] The window is now known to be slightly “south” of the main central galaxy bulge. The window is irregular in outline and subtends about 1 degree of the sky. It is centred on the globular cluster NGC 6522.[4]
Baade’s Window is the largest of the six areas through which central bulge stars are visible.[5]
Significance with respect to Astronomy in Botswana
The position of the Galactic Centre on the sky (as given by its ‘declination’ in the equatorial coordinates above) means that it passes directly overhead at some time every day at places on the Earth’s surface that are at latitude 29 degrees South of the equator. As the southern hemisphere at this latitude is mainly ocean, there are not many countries this latitude crosses (see map below).
This latitude is just south of Botswana in fact, but we are not far off. Its ‘Right Ascension’ (RA) in the equatorial coordinates above means that it is highest in the sky at midnight towards the solstice in June, which obviously is great for observations from Botswana (and all the other places on the map shown above) as it also coincides with the longest nights and generally a season of clearest and most stable weather.
Conversely, the maximum altitude of the Galactic Centre above the horizon gets progressively lower as one goes further North until at Northern European latitudes of around 53 degrees North, it is effectively never visible. Also, it hardly goes dark during summer solstice time in Northern Europe, so even if it were above the horizon it still wouldn’t be seen.
Other advantages for Botswana include very clear and dark skies compared to most places on Earth. It is also on average higher above sea level than Australia for example, which also helps because there is less atmosphere to be looking through.
It should be noted however that there are major international observatories in the Chilean Andes with very dark and much higher sites. For example, the long-established European Southern Observatory site at La Silla (used mainly for optical and infrared studies – https://www.eso.org/public/unitedkingdom/teles-instr/lasilla/) is at latitude 29.2 degrees South at an altitude of 2400m (highest point in Botswana is under 1500m). The newer ALMA Observatory (for sub-mm-wave studies – https://www.eso.org/public/unitedkingdom/teles-instr/alma/) is at around 23 degrees South and an altitude of over 5000m (one of the best observing sites in the World). There are several other major observatories in the Chilean Andes including Paranal (https://www.eso.org/public/teles-instr/paranal-observatory/) nearby to which the world’s largest optical telescope, the Extremely Large Telescope (ELT – 39m primary mirror), is nearing completion (see https://www.eso.org/public/unitedkingdom/teles-instr/elt/).
For radio astronomy however, Botswana does have the great geographical advantage of being next to the main development of SKA Phase 1 in SA and expansion of the array into Botswana seems the next logical step. Botswana’s sparse population makes it a very radio-quiet part of the World too.