The point of closest approach (the point at which two bodies are the closest) is called the periapsis or pericentre, from Greek, peri, around, and κέντρον. The point of farthest excursion is called the apoapsis apó, from, apocentre or apapsis before an unaspirated, or ἀφ-, aph-, before an aspirated vowel, respectively]), (the latter term, although etymologically more correct, is much less used). A straight line drawn through the periapsis and apoapsis is the line of apsides. This is the major axis of the ellipse, the line through the longest part of the ellipse.
Derivative terms are used to identify the body being orbited. The most common are perigee and apogee, referring to orbits around the Earth (Greek γῆ, gê, "earth"), and perihelion and aphelion, referring to orbits around the Sun (Greek ἥλιος, hēlios, "sun"). During the Apollo program, the terms pericynthion and apocynthion were used when referring to the moon.
The apoapsis of an orbit of one object around another is the point at which the one object is furthest away from the other object. Apoapsis is the general term for such a point, but there are also many specific terms for specific cases: the aphelion is the furthest point from the Sun in an orbit around the Sun. Likewise, apoastron is linked to other stars, apogee to the Earth, and apojove to Jupiter. The opposite is periapsis. The word apofocus is sometimes used instead of apoapsis, and apse instead of apsis.
The periapsis of an orbit of one object around another is the point at which the one object is closest to the other object. Periapsis is the general term for such a point, but there are also many specific terms for specific cases: the perihelion is the closest point to the Sun in an orbit around the Sun. Likewise, periastron is linked to other stars, perigeum to the Earth, and perijove to Jupiter.
A common misconception is that the sun is larger when it is near the horizon than when it is high overhead. However, this optical illusion is not true, for the apparent size of the sun is virtually the same when it is rising or setting near the horizon or when viewed overhead (in fact, it is very slightly smaller when viewed near the horizon due to refraction as well as the greater added distance in observing across the earth's radius). This illusion has been wrongly attributed to landmarks near the horizon, such as homes and trees, supposedly giving a sense of perspective and whereas the same perspective is lost when looking at the overhead sun bathed in an empty sky.
However, if we were approach the apparent size of the sun methodically by studying it during perihelion and aphelion, we can detect a small change using photographic equipment thanks to the elliptical orbit of our planet around the sun which leads to variations in distance (and apparent size) of the order of about 3.4%. More specifically, at perihelion each January, earth is approximately 147 million km away from the sun and whose apparent diameter is about 32.53' whereas, at aphelion each July, earth is approximately 152 million km away and the sun is characterized with an apparent diameter of about 31.46'. This difference of 5 million km between perihelion and aphelion leads to the slight change in the apparent diameter of the sun as illustrated by the two images of the sun below captured six months apart when the sun was near its minimum possible perihelion (Jan 2/2005) and maximum possible aphelion (Jul 5/2005) and while crossing the local meridian.
