This is indeed the June Newsletter even though you will not have received it before sometime in July.  Sorry I’m running late (and the web master is also late in posting this to the web site due to summer travel).  At our last meeting, June 12, we decided to hold only two more meetings in 2002: on Wednesday, September 11, and Wednesday, November 13.  Our policy of holding only 3 general meetings during the first half of the year seems to have worked, in that our attendance was higher than when we were meeting every month.  Maybe absence makes the heart grow fonder!
       We will continue sending a Newsletter each month, and we will also have a monthly star party.  The sky continues moving westward 15 deg every month whether we meet or not, so we’ll try to keep up with the changing panorama.  Our next Star Party will be Sunday, July 14 (see Coming Events).

       (Edited from This Is Oxfordshire; first published May 31, 2002.)  On November 10, last year, a man was hit and killed by a car in Oxfordshire, England, after a security light temporarily blinded the driver.  The accident victim was trying to cross a high-speed highway, A4074. 
       At an inquest in Oxford, the driver said he first saw a man a few feet from the front of the car.  The driver swerved and hit the brakes but couldn't avoid striking the man.  The driver said that a bright security light on a pub had obscured his vision.  The accident victim died shortly in an Oxford hospital.
       Because of the security light, a police constable who carried out a reconstruction of the accident said he could barely see an officer standing where the victim would have been.  The Oxfordshire coroner recorded a verdict of death as the result of an accident.  The security light has since been removed.

       (Edited from NASA.)  An ingenious arrangement of three homebuilt 14-inch telescopes on fixed mountings enables Tucson-based amateur astronomer, Roy Tucker, to conduct a backyard hunt for asteroids on a par with the best professional searches in the world.
       Tucker's scopes scan the sky as the Earth turns, reaching a magnitude of 20.5, fainter than most professional asteroid searches.  The three scopes produce sequential images that can be compared to reveal moving objects.  If you have any question whether we need more asteroid patrols, see the next article.

       (Edited from Sky & Telescopes Weekly News Bulletin, June 21, 2002.)  On June 17th, astronomers from the Lincoln Laboratory Near Earth Asteroid Research project (LINEAR) discovered a new earth-crossing asteroid.  The object is a little more than 100 yards across.  The asteroid flew between the earth and the Moon's orbit on June 14th, three days before it was discovered.  The object passed within about 75,000 miles of the earth, only about 9 times the diameter of the earth.  This is only the sixth known asteroid to penetrate the Moon's orbit, and by far the biggest.  The exact details of an impact scenario depend on the rock's composition.  Such an impact might be comparable to the 1908 Tunguska event in Siberia, with a force rivaling the largest H bombs.

       Now You See It, Now You Don't.  Beginning with a simple but instructive observation, it is easy to find a dim star that disappears when you look directly at it, and then reappears when you look away two or three degrees.  This is the familiar ploy of using averted vision (or averted imagination, as Doug McCombs calls it).  Some of the dim stars in the Little Dipper readily show this effect, and so does the Ring Nebula in a modest telescope. This effect is not due to the blind spot, described below.
       Figure 1 shows the distribution of rods and cones in a horizontal cross section of the human retina.  There are as many as 160,000 of these light receptors per square millimeter (a millimeter is about 1/25 of an inch).  There are about 130 million rods and 6 million cones in each human eye.  However, many of these receptors are connected with each other, so it would not be appropriate to suppose the human eye has the equivalent of 136 megapixels.
       The cones are highly concentrated in the fovea, the central 2 deg of the retina, the sensitive film-like portion of the eye.  The cones respond to color, but are less sensitive to dim light than are the rods.  The greatest concentration of rods is about 20 deg away from the visual axis, and there are no rods in the fovea.  Thus, a dim star may disappear in direct vision but reappear if you look a little away from the star.
       The Blind Spot.  Strangely, the rods and cones are located in the rear of the retina, pointing away from the incoming light.  The nerve fibers carrying information from the rods and cones are in the front of the retina.   These fibers come together to make up the optic nerve, which then transmits information to the brain.  There are no rods or cones where the optic nerve leaves the retina - hence the blind spot, an area of 5 deg or more where we can't see anything (see Figure 1).  Ordinarily, we are not aware of our blind spots, one in each eye, but you can easily lose things in your blind spots (see Figure 2).
      Visual Acuity is essentially fineness of vision.  Detection acuity is measured by finding the smallest object that a subject can just see, and resolution acuity is the smallest separation between two objects that can be seen as two.  Acuity is usually measured using black targets against a white background.  Young observers can detect targets of about 1 arcsec and resolve targets separated by about 1 arcminute.
       In astronomy, we overwhelmingly look at bright targets against the dark sky.  Furthermore, many of our targets, nearly all the stars, have no measurable angular subtense.  Thus in many cases, detection acuity reduces to the detection of brightness. Visual acuity closely follows the distribution of cones in Figure 1.
       The most important functions of a telescope are magnification and light gathering.  Both of these functions allow us to see things that we could not see otherwise.  For example, without magnification, Albireo, the head of the Swan, looks like a single star.  But in a modest scope, Albireo is perhaps the most beautiful double star in the sky, one member golden and the other blue. 
       The two members are about 34 arcseconds apart, below the naked-eye resolution threshold.  But with modest magnification, Albireo is clearly a double star.  Bigger telescopes  yield brighter images, finer resolution, and better color.
       Dark Adaptation.  Probably everyone has gone into a dark theater on a sunny day.  How well can you see when you first go in?  And how well can you see several minutes later?  After 15 or 20 minutes in the dark, we see much better, and our vision continues improving slowly for about 30 minutes, whether in the dark theater or under the night sky (see Figure 3). 
       The biphasic nature of the dark adaptation curve is curious, in that I have never found anyone who has had any subjective awareness of this aspect of the adaptation process. The first leg of the curve represents cone adaptation, so early in dark adaptation, people can see color.  But after about 7 minutes, only brightness can be detected at threshold, whatever color of light is presented.  Sensitivity of the rods, which are in effect colorblind, continues improving until about 30 minutes in the dark.
       Besides adaptation of the rods and cones, dilation of the pupil also plays a role in dark adaptation.  The pupil is the hole in the iris that lets the light into the eye, much like the function of the iris diaphragm in a camera.  The pupil varies from about 2 to 8 mm, but in older people, the pupil does not open as wide.
       We use red lights when we are observing because the rods are least sensitive to red and most sensitive to blue light, at the other end of the spectrum. The cones are most sensitive to yellow-green, in the middle of the spectrum, so a dim red light is the best night light overall for astronomers, or other people who need to preserve their dark adaptation.
       Depth and Distance Perception.  We have two eyes, separated in space, providing a basis for binocular depth perception.  When the two eyes fixate an object, the eyes converge on the object, each eye turning slightly inward.  By measuring the convergence angle and using the distance between the eyes as a baseline, the distance to an object can be found by triangulation.  A similar process led to our first measurement of the distance to a nearby star, by making two observations six months apart and triangulating across the earth's orbit.  In human vision, binocular disparity, the fact that each eye sees a slightly different view of the world, provides a stronger basis for depth perception.
       I showed two compelling displays in my presentation that I cannot reproduce here, a picture of craters on the moon, and a picture of the surface of Mars taken by our Voyager spacecraft.  Craters are more readily visible and compelling when they are lit at an oblique angle.  If you hold a picture of a crater so the sunlit was coming toward you, then the near side of the crater will be lighted and the far side will be in shadow;  the crater then looks like a crater.  But if you turn the picture upside down, then the near side is in shadow and the far side is lighted, and the crater looks like a hill.  The same considerations apply to the perception of topographic features on Mars. 
       The Moon Illusion.  Most people are aware that the moon looks much larger near the horizon than it does high overhead.  Many suppose this is some kind of optical effect, that somehow the image is larger near the horizon, but that is not the case.  In fact, an observer viewing the moon near the zenith is closer to the moon by approximately the radius of the earth, about 4,000 miles, than is an observer viewing the horizon moon.  Since the moon is about 230,000 miles from the earth, the image of the zenith moon is about 2% larger than the image of the horizon moon, but the zenith moon appears smaller.  The moon illusion is greatest near the time of the full moon.  To most observers, the horizon moon appears nearly half again as large as the zenith moon. There is a large amount of research on the moon illusion, but space limitations preclude further discussion here.  Check it out for yourself.  Seeing is believing - sometimes, anyway!