“One touch of nature makes the whole world kin” W. Shakespeare

1. 6-11-03 15:30h IFAE THURSDAY MEETING A Glance Around Xavier Portell “One touch of nature makes the whole world kin” W. Shakespeare

2. Outline • A glance to shadows • A glance to mirages • A glance to the sun • A glance to the moon • A glance to blue sky • A glance to halos • A glance to coronae • A glance to opposition of the sun • A glance to auroras A glance around

3. A glance to shadows Erathostenes of Cirene (276-196 BC) calculated the radius of the Earth using basically one vertical stick on the ground and measuring the shadow at different places. He obtained 12,800 km for the diameter. Real value is, in average, 12,739.71 km. Aristarchus also tried to calculate the distance of the Sun using the fact that when Moon is at its first quarter, a right-angled triangle can be drawn. The average distance calculated was a poor 8,000,000 km when, in reality, it is pretty close to 150,000,000 km. The difficulty was in angle measuring. Aristarchus of Samos (320-250 BC) calculated the distance Earth-Moon looking the shadow of the earth during the Moon eclipse. This method was improved by Hiparchus of Nicea (190-120 BC) and he obtained the distance 384,000 km (using the value of the earth radius calculated by Erathostenes). Real value is, in average, 384,317.2Km When looking around, one need to pay attention to shadows: much information can be extracted from them! A glance around

4. More shadows A glance around

5. Earth shadow... One can even see shadow of mountains! This is caused by the casting of Earth shadow on the atmosphere. The pinkish zone is called “The Belt of Venus” or “The Anti-Twilight Arc (backscattering of setting sun light). 6:45h 7:15h Arizona DASI (Degree Angular Scale Interferometer) in Antartica. View opposite the sun. Looking at the west on sunrise or eastwards on sunset, one can depict a pinkish zone in the sky separating a dark blue zone from the normal blue sky. A glance around

6. ...and “earthlight”! This effect was already noticed by Leonardo da Vinci. It is best seen a week before or after new moon (third or first quarter). One can see a bluish part on the moon due to the earth light: This nice feature has some applications on determining global warming!! The astronomer André-Louis Danjon, in the beginning of 20th century first collected some quantitative data from that event but it wasn’t until Steven E. Koonin in 1991 who explained the potentiality of this method to measure the albedo of the earth and to monitor the climate change. • New Jersey Institute of Technology and California Institute of Technology started collecting precise data and, in spring 2001, after 200 nights looking the sky, reported that earth albedo is 0.297±0.005. The technique is not expensive and they compensate for atmosphere scattering thanks to the bright moonlight. Some interesting things are also noticed: • Seasonal variation upto 20% • Variations due to different parts of the Earth (Asia continent) • A 2.5% decrease in the albedo over the past five years while the sun was increasing its activity. A glance around

7. A glance to mirages Inferior mirage Superior mirage Mock mirage A glance around

8. Green flash A glance around

9. Green flash explained The “apparent position” of the sun. The “real position” of the sun. When refracting, light is splited in different colors (like prism does). Shorter wavelengths are bent more dramatically. When the lowest part of the sun disappears, last rays are green and blue but the latest are scattered the most. One needs a calmed day and a low and stratified horizon to see this effect. During the sunset we see the sun thanks to refraction (0.5º is the diameter of the sun). Depending on the different layers that compose the atmosphere the sun can be much distorted. A glance around

10. Moon Illusion Angular diameter of the moon is about 0.5 degrees The Ponzo effect Our vision of the sky is a flattened one where things at the horizon seems further than things overhead (e.g. Airplanes). The Ponzo effect comes into play when assuming the moon stuck on the sky’s dome. • Another theory (Dr. Don Mc. Cready, Winsconsin, 1999). Two illusions: • Oculomotor micropsia: objects of smaller angular size when seems to be close to us based on distance cues. • Oculomotor macropsia: objects of bigger angular size when seems to be far from us based on distance cues. A glance around

11. Saturation of blue sky Rayleigh scattering is the responsible to the blue color of the sky: • Dependence on the frequency is the clue. • It is predominant with the presence of molecules and tiny particles (<1/10 ) • It is an elastic scattering. Mie scattering is mainly produced when the size of molecules is >1/10 . It has a sharper and more intense lobe in the forward direction and it is independent of the wavelength. A glance around

12. A glance to halos Halos are a relatively common event that happens in the sky and are due to crystals that exist in suspension in the atmosphere. Typically one can see them surrounding the sun or the moon. There are plenty of different types of halos and knowing a little bit about their formation can help to determine the structure of some clouds and to better admire their beauty. A glance around

13. Halo formation German Halo Research Group (Europe) Light enters to ice crystals and get reflected or refracted due to different media properties. The regularity of crystals is the responsible for the concrete shape of the output light. • The frequency of the different type of halos depend very much on three main things: • The quality of the crystals • The size and alignments • The size of the halo itself A glance around

14. Type of crystals Size of crystals Physical consequence Quality of halo ~0.01 mm Diffraction Weak & diffuse >0.05 mm Reflection and refraction Good >0.1 mm Reflection, refraction and alignment Very good and sharp >1 mm Poor optical quality Diffuse The size, alignments and optical quality of the crystal have a direct consequence in the halo formation. Poor quality Good quality A glance around

15. Halos Circumzenithal arc 22º halo Tangent arc Sun Pillar Sundog A glance around

16. 22º circular halo Disordered hexagonal crystals make light deviate in different directions. Only light passing through two of their side faces will deviate in a range from 22º to 30º (most of them at 22º). Different people see halos made from different crystals. As in rainbows and unlike diffraction, red light is reflected less than blue light. That explains the inner red color of the circle. There also exist moon halos but they are less bright. A glance around

17. Sundogs As horizontal plate crystals fall down in quiet air, they tend to align themselves as leaves falling from a tree. When the sun (or the moon) is low, one can see two points of higher intensity of light which are usually called sundogs (or moondogs). They are situated on the same line of the light source, at 22º. They are also called Mock Suns and, when the sun is higher, the position may vary thanks to reflections on down side of crystals. Rainbow colours are caused by dispersion Sometimes, reflection due to needle crystals provides enough surface for reflecting light into a huge parhelic circle! A glance around

18. Tangent arc This arc is more difficult to think but it is due to the deviation of light caused by hexagonal ice particles falling. The deviation of light is not only 20º but a continuous spectra up to 60º. An upper and a lower tangent arc can be seen and both can join at certain solar altitude to a complete arc (circumscribed halo). Red light is also less reflected than blue light and the different colors are sharper than in 22º halo thanks to the orientation of the crystals. A glance around

19. Pillars The crystals deviates downward the rays that come from the sun and an upper pillar can be seen. There exist also lower pillars caused by upward deviations. The weather must ve very cold to see this kind of pillars caused by artificial lights. There exist also pillars caused by the moon or even by artificial lights. A glance around

20. Circumzenithal arcs This kind of arcs happen when rays enter by one surface and get out from another kind of surface. The sun must be higher than the ice crystals but the situation is similar to the appearance of sundogs. The deviation of light is close to 90º hence the colors are much purer and separated than even the rainbows! The best conditions are when the sun is at 22º. If the sun is higher than 32.3º the light is internally reflected and it cannot be produced. When the light is internally reflected it scapes from the crystal through one of the horizontal faces and contributes to the parhelic circle A glance around

21. Examples A glance around

22. A glance to corona Diffraction is the cause of sun or moon corona. Light diffract with dropplets of water and several circles can be seen thanks to different wavelengths. The sharper they are, the similar in size the dropplets are. Some clouds can show iridescence. This is similar to the corona effect... A glance around

23. Opposition effects Specter of the Brocken Opposition effect (Without water dropplets) Glory (Mie scattering and diffraction) Heiligenschein (water nearly on the ground) A glance around

24. A glance to aurora A glance around

25. Aurora explained Aurora’s colors (green, red...) are caused due to the ionization of rarefied oxygen in the upper layers of the atmosphere (100km). Solar wind is the responsible for that effect. Magnetic field lines cannot cross each other and that’s why earth magnetic lines are pulled appart from the sun. The magnetopause is the boundary surface between interplanetary field lines and those of the Earth. There exist some polar cusps where the field must be zero and the energetic particles from the sun enter the atmosphere. The ions and electrons follows magnetic field lines and are trapped in the radiation belts. While approaching to Earth the intensity is higher and it exist the so called mirror effect that is also the cause for an electric current circling the Earth clockwise (as seeing from above). Voltages upon 15000 V accelerates particles that hit the atmosphere giving the colors observed. A glance around

26. ...there’s much to see and to learn from!!! So, keep looking around... A glance around