The Rotation of the Sun

Roland BONINSEGNA

Abstract

Observing the Sun is always a delicate operation, especially using an instrument with students. Some advices are presented to avoid problems. Sunspots drawing, using image projection onto a screen, is also described as first hand material for the determination of solar rotation.

I) Observation with the naked eyes: be careful!!

Could you look at the Sun with the naked eyes during several seconds? Probably not! You must avoid too much light, but intense sunlight for a short period time is harmless and probably beneficent (see Aldous Huxley). I usually enquire to my students about the true colour of the Sun. To verify their assumptions, I ask them to look directly at the Sun for no more than two or three seconds, in order to prevent for being too much dazzled. If direct observation for a very short period time is not dangerous, the problems arise when using non-adequate protecting material for longer observation time. Never use classical solar glasses, glass blacken by a flame, negatives from colour film. A filter used in soldering with a maximum index protection, developed black and white negatives are better shields.

The best way, however, is to use solar glasses, not the usual ones but those especially prepared for a solar eclipse, reducing 99.999 % of light intensity. With this protection, you should be able to discern the large sunspots and be aware of the rotation of the Sun. Some special events like total and partial solar eclipses, Mercury and Venus transits across solar disk, should be worth observing (see tables 1 and 2). Even when you owned this type of solar eclipse glasses, you must verify that the filters are not scratched or rubbed. The Mylar or, to a less extent, AstroSolarTM filters, with a silvery reflection, are the most delicate when compared to the polymer filters.

II) Using an astronomical instrument.

A) Projection method: be more than careful!!

The method is simple: projecting the Sun image onto a white screen avoids a direct look through the telescope and allows several persons to see the solar disk at once. It is the most convenient method with a group, but also the more dangerous one to use. You can use whatever instrument you want: refractor, reflector, and binocular. Cover the finder scope objective to prevent bad surprise. If possible, use a low quality eyepiece of medium magnification. First of all, if your instrument is equipped with an equatorial mounting, it is better to set it in the correct way. The main axis to horizon angle must be the same value that your local latitude. The main axis must also be aimed roughly at the geographical north. Never look through the instrument when pointing the Sun: use the telescope shadow projected to the ground and minimises it. When the alignment is good, you will see a bright spot appearing on the instrument shadow. You must be careful not to let students trying to look through the instrument: as it has no protection, eye damage would be guaranteed even if no sensation of pain can be perceived.

The screen (a white sheet of paper fixed on a hard stand) can be independent from the scope: held by hand or, better, fixed on a photographic tripod, but always perpendicular to the light beam. In that case, drawing sunspots will not cause vibrations to the instrument, but the screen will have to be often removed, due to Earth’s rotation. To avoid that effect, it is also possible to fix the screen to the telescope tube, but you will have to draw the sunspots with much care, to avoid bad vibrations. A motor driven telescope is more comfortable. More you move away the screen, the larger, but also the dimmer the image will be. A good distance compromise would be around 20-30 cm from the eyepiece. You can use a sheet of paper with a circle already drawn on it, to fit the solar image inside. If the circle has a diameter of 114 mm, one millimetre corresponds to one degree in longitude or latitude on the Sun. However, this is true for the centre only, because the image is a flat representation of a globe. You will find another example of a prepared screen in figure 3 into the chapter devoted to the rotation of the Sun.

If you are using the projection method to draw sunspots, it is very important also to determine the geographical main directions. It is an essential must when you plan to determine solar rotation using your own drawings, made by projection through the telescope. First of all, draw the spots when the projected image fits well into the pre-defined circle. Do it rapidly in checking the correctness of the alignment. If you use a screen separate from the scope, you must draw one spot at once before adjusting again the screen to compensate for the Earth’s rotation in order to draw the second spot...(you will probably not have enough patience to draw many spots!). Don’t forget to record the date and time of the drawing.

When it is done for the last spot (which could be also the first one), let the moving image of the Sun going away from the pre-defined circle for a while. Before it leaves the sheet of paper, draw rapidly the position of the last spot. After the observation, trace a line between the two positions of the same spot: that axe represents the east-west diurnal motion of Earth’s rotation. The perpendicular of that line is the north-south geographical axis. See figure 1. The solar rotation axis will not correspond to the geographical one. To determine it, see the figure 5 of the chapter devoted the rotation of the Sun.

When you will use several drawings, made on several days, you will have to gather, in chronological order, the first onto the second one and to perform a perfect alignment with the two axes. Then mark on the first drawing the place of the sunspot seen on the second drawing by transparency (do it on a luminous table or on the glass of a window). Repeat the operations again with the first drawing (where you collect all the spots positions) and the third one, until the last drawing.

B) Direct visual observation using adequate filtering.

That method is not recommended when you have to deal with an important group of students or if you have the intention to make sunspots drawings. However, it is safer when you use correct filtering. The best way to reduce the amount of solar light is to use an adequate filter in front of your instrument. One of the best and cheapest filters is the AstroSolarTM safety film from Baader. It is not difficult to construct a hand-made filter adapted to your instrument. The colour rendition of that kind of filter mimics the real one.

Some instruments are sometimes equipped with solar filter that has to be screw at the eyepiece. In that case, the solar energy input, multiply by the mirror or objective diameter, very near the focal point could, in some dramatic cases, break the filter. Avoid that kind of assembly especially for telescope larger than 10 cm, or use it behind a Herschel wedge, which eliminate an important part of the light. Don’t forget to cover also the finder scope or to construct its own filter.

During total Sun occultations by the Moon (also express as “Sun’s eclipses”) you can remove the protection of your instrument for only some tens of seconds in order to admire the splendour of the phenomena. But be careful to replace it in time and not to waste too much time with the material handling in spite of looking to this rare event.

C) Other methods.

It is not my intention to present other observing methods such as photography or CCD. These are expensive and are best suited to use with a very limited number of students. However, the pictures created with these techniques are very interesting for teachers who have not the possibility to do observations with their students. For more information concerning how to find such documents, please consult the bibliography at the end of the chapter devoted to the solar rotation.

Bibliography.

Baader AstroSolar TM Safety Film, Baader Planetarium, Zur Sternw., D 82291 Mammendorf

Gray B.J., 2002, Guide 8 software,

Martres M.J.et al., 1987, Astronomie le guide de l’observateur, tome 1,17

Martres M.J.et al., 1994, The observer’s guide to astronomy, volume 1,1, Cambridge Univ. Press

Table 1: Future solar eclipses visible from somewhere in Europe. Date is in the DD/MM/YYYY format. The time is only indicative.

Table2: Future transits of inner planets across the Sun’s disk visible from Europe. These events are well suited for naked eyes observation using appropriate filtering.

N

W

A’

A

E

S

Figure 1: Determination of the main geographical axis using the natural apparent motion of a sunspot (A-A’) due to Earth’s rotation. The image presented is oriented as it was observed directly in the sky. The resulting image from a telescope view by projection onto a screen, could be oriented in a different way, but the direction of the Sun apparent motion remains the same: from East to West.