question: Why can we look at the sun when it's rising or setting without hurting our eyes, but when it's miday we can't look at it without hurting our eyes.
comment: At mid day sunlight is coming from nearly directly above you, and goes through approximately 2 miles (4km) of atmosphere (air), as the photons - light particles from the sun hit air molecules like ozone and water, they absorb some of the radiation or energy of the sun light and also scatter some of them away from your line of vision, but yes enough high energy photons come through that 2 miles or so to do damage to your eyes if stared at directly for more than a second or two. & #x200B; At sunrise/sunset, the angle of the light travels through a little more atmosphere. For a six-foot tall person, the horizon is a little more than 3 miles (5 km) away, so that extra mile of air scatters the light more and absorbs more of the photons. & #x200B; A good way to think of it is looking at a light bulb in a lamp directly may hurt your eyes, but if put your hand up and cover half the light bulb, it is not as bright.
passages: Section::::The Sun.
  
  From the tropics and middle latitudes, the Sun is visible in the sky at its upper culmination (at solar noon) and invisible (below the horizon) at its lower culmination (at solar midnight). When viewed from the region within either polar circle around the winter solstice of that hemisphere (the December solstice in the Arctic and the June solstice in the Antarctic), the Sun is below the horizon at both of its culminations. 
   Indirectly scattered sunlight comes from two directions. From the atmosphere itself, and from outer space. In the first case, the sun has just set but still illuminates the upper atmosphere directly. Because the amount of scattered sunlight is proportional to the number of scatterers (i.e. air molecules) in the line of sight, the intensity of this light decreases rapidly as the sun drops further below the horizon and illuminates less of the atmosphere.
   The brightness of the Sun can cause pain from looking at it with the naked eye; however, doing so for brief periods is not hazardous for normal non-dilated eyes. Looking directly at the Sun causes phosphene visual artifacts and temporary partial blindness. It also delivers about 4 milliwatts of sunlight to the retina, slightly heating it and potentially causing damage in eyes that cannot respond properly to the brightness. UV exposure gradually yellows the lens of the eye over a period of years, and is thought to contribute to the formation of cataracts, but this depends on general exposure to solar UV, and not whether one looks directly at the Sun. Long-duration viewing of the direct Sun with the naked eye can begin to cause UV-induced, sunburn-like lesions on the retina after about 100 seconds, particularly under conditions where the UV light from the Sun is intense and well focused; conditions are worsened by young eyes or new lens implants (which admit more UV than aging natural eyes), Sun angles near the zenith, and observing locations at high altitude.
   Looking directly at the photosphere of the Sun (the bright disk of the Sun itself), even for just a few seconds, can cause permanent damage to the retina of the eye, because of the intense visible and invisible radiation that the photosphere emits. This damage can result in impairment of vision, up to and including blindness. The retina has no sensitivity to pain, and the effects of retinal damage may not appear for hours, so there is no warning that injury is occurring.
   Towering and stooping are more complex forms of atmospheric refraction than looming and sinking. While looming and sinking change the apparent elevation of an object, towering and stooping change the apparent shape of the object itself. With towering, objects appear stretched; with stooping, objects seem to be shortened. The apparent stretching and shortening of the objects are not symmetrical and depends on the thermal profile of the atmosphere. The curvature of the rays changes more rapidly in some places because the thermal profile is curved.
  
  Section::::Image example and explanation.
   On Earth, an average night lasts shorter than daytime due to two factors. Firstly, the Sun's apparent disk is not a point, but has an angular diameter of about 32 arcminutes (32'). Secondly, the atmosphere refracts sunlight so that some of it reaches the ground when the Sun is below the horizon by about 34'. The combination of these two factors means that light reaches the ground when the center of the solar disk is below the horizon by about 50'. Without these effects, daytime and night would be the same length on both equinoxes, the moments when the Sun appears to contact the celestial equator. On the equinoxes, daytime actually lasts almost 14 minutes longer than night does at the Equator, and even longer towards the poles.
   After the Sun, the second-brightest object in the Mercurian sky is Venus, which is much brighter there than for terrestrial observers. The reason for this is that when Venus is closest to Earth, it is between the Earth and the Sun, so we see only its night side. Indeed, even when Venus is brightest in the Earth's sky, we are actually seeing only a narrow crescent. For a Mercurian observer, on the other hand, Venus is closest when it is in opposition to the Sun and is showing its full disk. The apparent magnitude of Venus is as bright as −7.7.
   Whilst the last members were signing it Doctor. Franklin looking towards the Presidents Chair, at the back of which a rising sun happened to be painted, observed to a few members near him, that Painters had found it difficult to distinguish in their art a rising from a setting sun. I have said he, often and often in the course of the Session, and the vicissitudes of my hopes and fears as to its issue, looked at that behind the President without being able to tell whether it was rising or setting: But now at length I have the happiness to know that it is a rising and not a setting Sun.
   BULLET::::- The intensity of UV radiation reaching the surface of the earth depends on the angle of the Sun in the sky. Each day, the Sun achieves its highest angle (highest intensity, shortest shadows) at solar noon, which only approximately corresponds to 12:00 on clocks or 13:00 during daylight saving time. This is because of the differences between solar time and local time in a given time zone. UV risk is high when the Sun is directly enough overhead that people's shadows are shorter than their height.
   The sky's brightness varies greatly over the day, and the primary cause differs as well. During daytime, when the Sun is above the horizon, the direct scattering of sunlight is the overwhelmingly dominant source of light. During twilight (the duration after sunset or before sunrise until or since, respectively, the full darkness of night), the situation is more complicated, and a further differentiation is required.
   BULLET::::1. From the Earth, the Sun appears as a disc rather than a point of light, so when the centre of the Sun is below the horizon, its upper edge may be visible. Sunrise, which begins daytime, occurs when the top of the Sun's disk appears above the eastern horizon. At that instant, the disk's centre is still below the horizon.
  
  BULLET::::2. The Earth's atmosphere refracts sunlight. As a result, an observer sees daylight before the top of the Sun's disk appears above the horizon.
   When the Sun is low above the horizon, sunlight rays must penetrate the atmosphere for a greater distance, reducing the intensity of the direct light, so that more of the illumination comes from indirect light from the sky (Thomas 1973, 9–13), reducing the lighting ratio. More blue light is scattered, so if the Sun is present, its light appears more reddish. In addition, the Sun's low angle above the horizon produces longer shadows.
   BULLET::::5. The distribution of atmospheric seeing through the atmosphere (the C profile described below) causes the image quality in adaptive optics systems to degrade the further you look from the location of reference star
   As sunrise and sunset are calculated from the leading and trailing edges of the Sun, respectively, and not the center, the duration of a daytime is slightly longer than nighttime (by about 10 minutes, as seen from temperate latitudes). Further, because the light from the Sun is refracted as it passes through the Earth's atmosphere, the Sun is still visible after it is geometrically below the horizon. Refraction also affects the apparent shape of the Sun when it is very close to the horizon. It makes things appear higher in the sky than they really are. Light from the bottom edge of the Sun's disk is refracted more than light from the top, since refraction increases as the angle of elevation decreases. This raises the apparent position of the bottom edge more than the top, reducing the apparent height of the solar disk. Its width is unaltered, so the disk appears wider than it is high. (In reality, the Sun is almost exactly spherical.) The Sun also appears larger on the horizon, an optical illusion, similar to the moon illusion.
   Sailors can take reliable star sightings of well-known stars, during the stage of nautical twilight when they can distinguish a visible horizon for reference (i.e. after astronomic dawn or before astronomic dusk). 
  
  Under good atmospheric conditions with the absence of other illumination, during nautical twilight, the human eye may distinguish general outlines of ground objects but cannot participate in detailed outdoor operations.
   Under normal conditions, the Sun is so bright that it is difficult to stare at it directly. However, during an eclipse, with so much of the Sun covered, it is easier and more tempting to stare at it. Looking at the Sun during an eclipse is as dangerous as looking at it outside an eclipse, except during the brief period of totality, when the Sun's disk is completely covered (totality occurs only during a total eclipse and only very briefly; it does not occur during a partial or annular eclipse). Viewing the Sun's disk through any kind of optical aid (binoculars, a telescope, or even an optical camera viewfinder) is extremely hazardous and can cause irreversible eye damage within a fraction of a second.
   After the sun has also set for these altitudes at the end of nautical twilight, the intensity of light emanating from earlier mentioned lines decreases, until the oxygen-green remains as the dominant source.
  
  When astronomical darkness has set in, the green 557.7 nm oxygen line is dominant, and atmospheric scattering of starlight occurs.
  
  Differential refraction causes different parts of the spectrum to dominate, producing a golden hour and a blue hour.
  
  Section::::Relative contributions.
   The effects of atmospheric seeing are qualitatively similar throughout the visible and near infra-red wavebands. At large telescopes the long exposure image resolution is generally slightly higher at longer wavelengths, and the timescale ("t" - see below) for the changes in the dancing speckle patterns is substantially lower.
  
  Section::::Measures.
  
  There are three common descriptions of the astronomical seeing conditions at an observatory:
  
  BULLET::::- The full width at half maximum (FWHM) of the seeing disc
   Typically an observer attempting a Messier marathon begins observing at sundown and will observe through the night until sunrise in order to see all 110 objects. An observer starts with objects low in the western sky at sunset, hoping to view them before they dip out of view, then works eastward across the sky. By sunrise, the successful observer will be observing the last few objects low on the eastern horizon, hoping to see them before the sky becomes too bright due to the rising sun. The evening can be a test of stamina and willpower depending on weather conditions and the physical fitness of the observer. Particularly crowded regions of the sky (namely, the Virgo Cluster and the Milky Way's galactic center) can prove to be challenging to an observer as well, and a Messier marathon will generally budget time for these regions accordingly.
   Before nautical dawn and after nautical dusk, sailors cannot navigate via the horizon at sea as they cannot clearly see the horizon. At nautical dawn and nautical dusk, the human eye finds it difficult, if not impossible, to discern traces of illumination near the sunset or sunrise point of the horizon ("first light” after nautical dawn but before civil dawn and "nightfall" after civil dusk but before nautical dusk).
   Because of the obliquity of the Earth's axis relative to the ecliptic, away from the equator some signs take much longer to cross the eastern horizon than do others. In the northern hemisphere, the signs from Cancer through Sagittarius take longer to ascend than the signs from Capricorn through Gemini, whilst in the southern hemisphere the reverse is the case.
   Sinking is the opposite of looming. In sinking, stationary objects that are normally seen above the horizon appear to be lowered, or may even disappear below the horizon. In looming, the curvature of the rays is increasing, while sinking produces the opposite effect. In general, looming is more noticeable than sinking because objects that appear to grow stand out more than those that appear to shrink.
  
  Section::::Towering and stooping.
   The angle of a celestial body with the zenith is the "zenith angle" (in astronomy, commonly referred to as the "zenith distance"). A body’s angular position can also be given in terms of "altitude", the angle above the geometric horizon; the altitude formula_13 and the zenith angle formula_14 are thus related by
  
  Atmospheric refraction causes light to follow an approximately circular
  
  path that is slightly longer than the geometric path, and the air mass must
  
  take into account the longer path (Young 1994).
  
  Additionally, refraction causes a celestial body to appear higher above the
   Due to refraction of light rays in the Earth's atmosphere the Sun will be visible above the horizon even when its disc is completely below the limb of the Earth. Additionally, when seen from the Earth, the Sun is a bright disc in the sky and not just a point of light, thus sunrise and sunset can be said to start several minutes before the sun's geometric center even crosses the horizon, and extends equally long after. These conditions produce differentials of actual durations of light and darkness at various locations on Earth during an equinox. This is most notable at the more extreme latitudes, where the Sun may be seen to travel sideways considerably during the dawn and evening, drawing out the transition from day to night. At the north and south poles, the Sun appears to move steadily around the horizon, and just above the horizon, neither rising nor setting apart from a slight change in declination of about 0.39° per day as the equinox passes.
   From the March equinox to the September equinox, the Sun rises within 23.5° north of due east, and sets within 23.5° north of due west. From the September equinox to the March equinox, the Sun rises within 23.5° south of due east and sets within 23.5° south of due west. The Sun's path lies entirely in the northern half of the celestial sphere from the March equinox to the September equinox, but lies entirely in the southern half of the celestial sphere from the September equinox to the March equinox. On the equinoxes, the equatorial Sun culminates at the zenith, passing directly overhead at solar noon.
  