Recently, an asteroid was named after Günther:
Eclips 4 januari 2011
On January 4th, at sunrise, the Netherlands was treated to a partial solar eclipse. Unfortunately, most of us saw very little due to heavy cloud cover. However, perseverance wins the prize! Günther wrote us:
I had better luck than most with viewing the eclipse. Even though chances of seeing anything were very small, I went out to the Grote Melm, here in Soest near the Eem, where there is an unobstructed view toward the southeast. Sure enough, very close to the horizon, there was a small strip of nearly clear skies, where you could see the eclipse as the Sun rose! 
The clear band was not much more than the half of the Sun's diameter, but for about 30 seconds, the view was outstanding and the eclipse was visible. I even made a quick photo with my camera (image above). The Sun and the eclipse is partially obscured by a distant stream of clouds. Other than that, I didn't see the Sun during the eclipse, but even this was worth the trip!
On the infrared photo from the eclipse day (image left), you can see that at sunrise and the moment of the eclipse, there was a narrow cloud-free zone just across the border with Germany. Because of this, from parts of the Netherlands (including Soest, which was about 80 km from the clearing), de sky in the SE had a clear stroke near the horizon, about 0.3 degrees high, through which you could just briefly see the rising eclipse!
Rainbows
Sun and rain: turn your back to the sun and you see the rainbow. He forms a circle with the shadow of your head as centre. So: the higher the sun in the sky, the smaller is the rainbow segment that pears over the horizon. In the summer you will never see a rainbow at midday, even if it pours rain – the entire bow is below the horizon. But in the afternoon it can be splendidly visible!
Just outside the bow you may notice the 2nd rainbow: it is weaker and its colours are reversed! This bow is the reflection of the 1st bow it the drop it selves: light undergoes not one, but two internal reflections at the wall of the drops. Between the 1st and 2nd rainbow the sky is darkest.
A rainbow is not a stand-alone phenomenon, but is the boundary between an area in the sky that is lit by a specific ray path through a drop. The closer one looks toward that boundary, the brighter is the sky – the outer boundary occurs for red light. Consequently it is brighter inside the 1st bow and outside the 2nd bow than in between. Painters which are familiar with this structure have often difficulties to realistically paint a rainbow.
(photo Ralf Pitchenender, München 8 July 2005)
A 3rd rainbow (yet another reflection) should also be possible, but that one appears at the sun facing side of the sky where the sky light is so bright that the bow submerges – you will never see it. However, occasionally one notices just within the 1st bow additional narrow bows, the so-called ‘supernumerary bows’. Their mutual distances relates with the sizes of the rainbow-making drops.
Within the 1st rainbow, the supernumerary bows are visible.
(photo Mike Nicholson, Papatoetoe, New Zealand 7 September 2007)
Rain during holidays: hardly any tourist is looking forward to that. But if it is doomed to happen, the beauty of a rainbow offers some comfort. And the longer one looks to it, the more beautiful it becomes! Therefore below a few extra pictures of remarkable rainbows:
A ‘rainbow exote’: the rainbow crosses its reflection bow, that is: the bow generated by the reflection of the sun in still water. Rare, but at still water, rain and low sun such a thing should occasionally be visible from the deck of a ferry crossing a shallow sea.
(photo Nicola Boll, Isfjord bij Spitzbergen 22 August 2006)
‘Broken rainbow’: the rainbow below the horizon is in sea water spray; the upper part in (fresh) rainwater drops. As the index of refraction of salt water is slightly larger, its rainbow is smaller. This picture proves the saltiness of seawater. Obviously you can also find out by just tasting the water
(photo J. Dijkema)
The answer to the frequently asked question ‘Why is the rainbow round?’ is simply: ‘Because a raindrop is round!
(photo Galen Rowell, Kauai Island, Hawaii 1993)
Island climate
Everyone feels it and everyone knows it: the climate of a small island is not like that of the inland. The differences: there is more wind (that’s logical), more sun and less rain. But does that impression match reality? The observations of the Dutch Met Office tell us that there 5% less rain and 1% less sun on a Dutch Frisian island (like Terschelling) than in the inland. This is way too small to be observable in the daily experience. Something seems wrong.
Just like elsewhere in the Netherlands, the weather on the islands may be spoiled by rain. But on warm summer days that does not happen so often as in the mainland.
In order to get a finger behind the island climate, we first consider the weather situations in which there is no difference between island and inland. This is the case if we are under the influence of oceanic depressions, which transport cool air – we talk about the summer – with in it the mighty Atlantic rain fronts into Europe. These large-scale weather systems are hardly influenced by the nature of the terrain below them and therefore bring everywhere the same weather: rainy and cloudy alternated by spells, and moderated temperatures. In the Netherlands most days are characterized by this type of weather.
Much rarer are the days where the climate turns to us its other face. These are the days that we are under the influence of a high pressure system that normally belongs in Southern Europe. In that situation the ocean depressions are kept on distance and the temperatures (again, I am talking about the summer) are high – particularly in the inland. But when the sun pushes the temperature difference between soil and upper air above a certain threshold, then clouds are formed which may grow out in the afternoon to thunderstorms, to disappear in the late evening again. From the Dutch islands one regularly sees such thunderstorms floating over the main land.
In such situations, the weather on the islands is different: while the inland, consistent with the weather forecast, suffers under an developing cloud deck, the island enjoys a beautiful and sunny day. For, the (hot) surface of the island is so small that the cloud formation gets no chance to get on steam: at the moment that a clouds tries to develop towards a thunderstorm, they find themselves already situated over the cool seawater and hence fade away.
In the coastal areas of the mainland a similar situation may occur, so that it is there also sunnier during warm days than in the inland. The difference with an island is that there is only on one side sea, so that the island/coastal climate is less prominently present than on a sea-surrounded island.
The irony of all this is that the coastal/island climate does not manifest itself during drizzly days when everybody sits home, but rather on the (rarer) days when the weather tends to be fine: then it is on the island real fine. This is the very reason that we really know the island climate from our own experience.
On seasons other than the summer the island knows also specific climate characteristics. Perhaps the finest example is in fall, when the land is colder than the surrounding sea. Over sea may, in high pressure situation like above, develop thunderstorms. In this case not because of excess warming from below, but because of night-time cooling on the tops of the clouds. In am rather sure that island visitors regularly witness during the late night a spectacular thunderstorm over the North Sea.
Algol: my favorite naked-eye object
Algol is the famous ‘blinking star’: it is once every three days weaker than normal. If one is used at Algol’s normal appearance, it is quite impressive to see it in its minimum. Its brightness is then comparable to that of the little star just next to him (r Per). Its peculiarity stems from the fact that Algol is a binary system that undergoes mutual occultations. If the largest component occults the weaker one, the total brightness decreases by a factor 3, from magnitude 2.1 tot 3.4. Visually the process takes 8 hours: first the brightness continuously decreases for 3 hours; then it stays for 2 hours on its lowest value; finally the brightness increases in 3 hours back to normal.
In the course of a month the observable minima occur in clusters. Algol’s period is 3 days minus 3.2 hour, thus consecutive minima happen at earlier times of the day: when a certain minimum occur at midnight, the next one occurs three days later on 21 o’clock, and another one three more days later on 18 o’clock. After that, the minima occur at daytime and are not observable. But after about 8´3 = 24 days the minima shifted back by a full day, so after 23 days it occurs at midnight again. Hence, once every three weeks we get the opportunity to see Algol a few time in its minimum. If we restrict our available observation time to the period between sunset and midnight (that probably makes sense), then a series consists of 3 consecutive cases at most, all of them separated by three days. During the short summer nights the series are usually shorter – mostly consisting of just one case.
Not only is it worthwhile to observe Algol during its minimum brightness, but it is also neat to follow its brightness increase from minimum to normal. This process takes about 4 hours. In the winter season such an observation is easily doable – extra equipment is not needed. To me, this type of observation makes Algol one of the nicest objects in the sky. Every evening when I walk outside, I briefly check whether I can catch him on its minimum. Simple naked-eye astronomy, but most appealing indeed!
The next maximum can be found here. Set your alarm when it happens tonight!