When both Sun and Moon are visible why is the illuminated portion of the Moon not bisected by the line joining the Moon's centre to the Sun's?

Answer 1

I believe you are attempting to understand why the appearance of the moon at such times seems impossible; from the apparent location of the sun, the shadow "line" on the moon should appear to be perpendicular to a line connecting the moon's center and the sun's center. The answer is that during casual viewing we don't have a good way to observe or comprehend how really large and far away the sun is. In fact, the sun is so far away that for all practical purposes it is infinitely far away. As a result, the rays of sunlight hitting the moon are parallel to the rays of sunlight hitting the earth. The face of the sun is over 860,000 miles across. The distance between the earth and the moon is only roughly 240,000 miles. Several earth-moon spans can easily fit across the sun's face. Even though the sun is about 93,000,000 miles away, it is NOT a pinpoint of light that angles out to strike the moon and the earth at different angles. And it isn't NEARLY as close as our limited earth-bound experience leads us to believe. This gives us a very distorted perception of the earth-sun-moon angle. You can illustrate this in a fun way using Scott's Lunar Phase-ometer (OK, I made up the name). When the moon and the sun are both in the sky, stand where you can see the moon, and where you are in direct sunlight. Take a tennis ball or ball of similar size. Stick a stick through it, or suspend it from a string. While looking directly at the moon (NEVER try to look directly at the sun) hold the ball up at arm's length to cover the moon, and make sure you can see direct sunlight hitting the ball. You will notice that the shadow on the ball will duplicate the shadow on the moon. This can only be so if the light source is hitting the moon and the ball at the same angle (hence the idea that the sun is infinitely far away in a practicalsense). You can also speculate about where on the ball would be your position relative to your position on earth. This might also shed light on the observed position of the moon's illuminated part. Related to this question of the sun's distance, you have probably noticed as have most people in the world, times when sunlight seems to be streaming majestically down in beams that fan out widely from openings in the clouds, as glory from on high embracing the earth. Not. What you are observing is the sunlight coming straight toward you. There is no physical reference to help you actually see it this way; the illusion is that the sun is an orb a few comfortable miles above the earth, and the sun is reaching downward and out. It is like the painted lines on the sides of a long, straight stretch of highway. When you stand in the center (with friends watching for traffic of course) the lines do appear to converge as you look in the distance.

Answer 2

The illuminated portion of the Moon is not always bisected by the line connecting the Moon's center to the Sun because the relative positions of the Earth, Moon, and Sun create different angles that affect the illumination pattern. The phases of the Moon result from these changing angles, causing the illuminated portion to appear different from our perspective on Earth.