Flatness of the setting Sun Z. N´eda and S. Volk´an Babe¸s-Bolyai University, Dept. of Physics, RO-3400, Cluj, Romania E-mail: [email protected] (Last revised May 28, 2018) Atmospheric refraction is responsible for the bending of light-rays in the atmosphere. It is a result of the continuous decrease in the refractive index of the air as a function of altitude. A well-known consequence of this phenomenon is the apparently elliptic shape of the setting or rising Sun (or full-Moon). In the present paper we systematically investigate this phenomenon in a standard atmosphere. Theoretical and numerical calculations are compared with experimental data. The asymmetric rim of the Sun is computed as a function of its inclination angle, observational height and meteorological conditions characterized by pressure, temperature and lapse-rate. We reveal and illustrate some extreme and highly unusual situations. I. ATMOSPHERIC REFRACTIONS conditions was recently published by Thomas and Joseph [4]. Here we plan a more complete analysis, considering Atmospheric refraction is between the first scientifi- three different theoretical methods, experiments, com- cally approached phenomenon. This effect is responsible puter simulations and illustration of the results with a for the apparent scintillation of stars, mirages, the puz- collection of pictures and movies. zling and spectacular green flash, difference between ap- The spectacular nature of sunset or sunrise makes the parent and real position of stars or the asymmetric rim of phenomenon attractive to students. After our experience the Sun near the horizon. The refractive index of dry air it can effectively be used to exemplify refraction phenom- is very close to 1. Its small dependence on temperature ena and exercising the principles of geometrical optics. and pressure leads to a refractive index gradient in the Our paper is structured as follows. First, we introduce atmosphere. Although this gradient is very small, due to the atmosphere model, and the refractive index profile the large distances travelled by the light-rays in the at- is given. Then, we present three different methods for mosphere, it can result observable bending or dispersion. computing the ray-path in this atmosphere. Results com- Atmospheric refraction is usually divided into three puted for different meteorological conditions and observa- categories: astronomical, terrestrial and geodesic. Ter- tion altitude will close the theoretical part. Experimen- restrial refraction appears when both the object and ob- tally, we analyze and compare with theory the measured server are within the Earth’s atmosphere. This refraction flatness for some sunsets photographed or video-filmed is responsible for ordinary mirages, and has been exten- by us. Finally, we discuss some extreme and unusual sively studied [1]. The first one reporting and account- conditions which are exemplified by pictures and movies ing for mirages was Aristotle in Meteorologica [2]. Cor- on a web-page supporting this study. From this web- rectly, he concluded that dense air layers can act as mir- page one can also freely download a computer-program rors, and considered this effect responsible for mirages. written by us, which visualizes the sunset for arbitrary (A complete historic and bibliographic study of mirages meteorological conditions and observation altitude. can be found on the splendid web-page of A.T.Young We emphasize here, that our study is restricted to stan- [3].) Geodesic refraction is a special case of terrestrial dard atmospheric conditions with smooth temperature refraction where both the object and observer are at low and pressure profile. Non-standard, but often encoun- altitudes. A well-known example for this is surveying. tered atmospheric profiles leads to non-standard distor- We speak about astronomical refraction when a terres- tions of the solar rim, and are not considered within this trial observer detects ray-bending effects for the light- study. rays coming from objects outside the Earth’s atmosphere. arXiv:physics/0204060v1 [physics.pop-ph] 19 Apr 2002 This refraction is responsible for the difference between the real and apparent position of stars near the horizon, II. THE OPTICAL ATMOSPHERE MODEL the green flash or the asymmetric rim of the setting (or (REFRACTIVE INDEX AS A FUNCTION OF rising) Sun. In the present paper we discuss in detail ALTITUDE) the flatness of the Sun or full-Moon near the horizon. Particularly, we are interested in the extreme values this The atmosphere of the Earth is composed mainly of N2 flatness can take for standard atmospheric profiles, and (79%) and O2 (20%), and extends up to a few hundreds the dependence of the flatness on the altitude of the ob- kilometer height. The refractive index of air is very close server and meteorological conditions (temperature and to 1, and depends slightly on its pressure and temper- pressure). A similar study, considering fixed atmospheric ature following Edlen’s semi-empirical law [5], valid for 1 dry air: is the U.S. Standard Atmosphere, established in 1953 and re-actualized in 1976 [6]. The U.S. Standard At- − − P n =1+10 6(776.2+4.36 × 10 8ν2) . (1) mosphere consists of single profiles, representing the ide- T alized steady-state atmosphere for moderate solar activ- ity. The listed parameters include temperature, pressure, In the above formula ν is the wave-number of the light in − density, gravitational acceleration, mean particle speed, cm 1, P is the pressure in kPa and T the temperature mean collision frequency, mean free path, etc. as a func- in K. Since the pressure and temperature varies within tion of altitude. In our study we consider an atmosphere the atmosphere, we get a refractive index gradient which model with a spherical symmetry, all relevant physical is responsible for atmospheric refractions. Although the quantities (temperature and pressure) varying only as a variations in n are quite small, the large distances trav- function of altitude. We then calculate the refraction in- elled by light-rays in the atmosphere makes refraction dex of air as a function of altitude as follows: effects observable and sometimes important. (i) The temperature profile in the troposphere is linearly decreasing with a λ =6.5K/km lapse-rate, as suggested by the U.S. Standard Atmosphere. Following again the Ionosphere U.S. Standard Atmosphere we consider the temperature 100 constant within the tropopause. Although the temper- 0.001 ature increases as a function of altitude in the strato- Height(km) 80 0.01 Mesosphere sphere, due to the rarefied air (small pressure) the re- 0.1 60 fractive index is close to 1, and for the sake of simplicity Pessure(hPa) we assume the temperature as constant in this region, 1 OzoneLayer Stratosphere 40 too. (We checked that this approximation is fully jus- 10 Tropopause tified.) Above zs = 50km height we assume that the 20 refractive index is 1, and do not calculate it anymore by 100 Troposphere Edlen’s formula. Up to the top of the stratosphere the 1000 -80 -60 -40 -20 Ocean 20 0 temperature profile is therefore presumed as Earth Temperature(C)o FIG. 1. Distinct layers of the atmosphere, and the standard T (z)= T0 − λz for z<zt (2) temperature and pressure profile. T (z)= T0 − λzt for zt ≤ z ≤ zs, (3) As a function of altitude several layers with different where z is the altitude from sea-level, and T0 the tem- physical properties are distinguishable. The lowest layer perature at sea-level. extending from sea-level to approximative zt = 14km (ii) For the pressure profile we use a barometric formula height is called the troposphere. This is the region where in which we take into account the variation of tempera- the wheatear takes place, i.e. the region of rising and ture with altitude. Considering a vertical slice of air with falling packets of air. In this layer the air pressure drops thickness dz, the variation of pressure within this slice is drastically, at the top of the troposphere being only 10% due to the hydrostatic pressure of the value measured at sea-level. In the troposphere the temperature decreases almost linearly with the altitude dP = −ρ(z) g(z) dz, (4) (Fig. 1). A thin buffer zone between the troposphere and the next layer (the stratosphere) is called the tropopause. where g(z) is the gravitational acceleration and ρ(z) the Within the tropopause the temperature is in good ap- density of air at height z: proximation constant. The stratosphere extends from the M Nm P (z)m altitude of approximately 18km up to zs = 50km (Fig. 1). ρ(z)= = = . (5) Within the stratosphere the air flow is mostly horizontal. V V kT (z) In the upper part of the stratosphere we have the ozone (We denoted by m the mass of one molecule, N the num- layer. Inside the stratosphere the pressure decreases fur- ber of molecules in volume V , T (z) the temperature at ther with the altitude, but surprisingly the temperature height z and k the Boltzmann constant.) Since we fo- increases with height. Above the stratosphere we find cus on the troposphere and the stratosphere only, the the mesosphere and the ionosphere. In these regions the z height is small in comparison with the radius of the air is very rare and the temperature profile is shown in Earth (R ≈ 6378km), thus the g gravitational acceler- Fig. 1. E ation can be considered constant. We can write thus From the viewpoint of atmospheric refractions only the first two layers, the troposphere and the stratosphere are P (z)mgdz dP = − , (6) important. In the upper layers of the atmosphere the air kT (z) is so rarefied, that the refractive index can be considered 1 within a good approximation.