A close approach occurs not only between two planets, such as when you see Jupiter and Saturn in the night sky and they are within a few arc-degrees of each other but a close approach can occur between any two or more celestial objects, which includes; the moon, planets, dwarf planets, stars, asteroids, comets, meteors, and more.

Furthermore, A close approach between any celestial objects would always appear from the viewers perspective and which come to within a predetermined circle of close proximity.

The occurrences of any and all close approach (closest approach) events between two or more celestial object are all apparent phenomena caused entirely by the viewers perspective, as there is never any actual physical approach in space between the celestial objects involved.

However, the precise moment when two celestial objects have reached their closest approach is called the appulse. Many close approaches and the subsequent appulse, when the event involves two bright celestial objects are naked-eye events which often attract wide public interest for general observation.

In astronomy, naked-eye events are those when the eyes and only the eyes, unaided with any form of optical enhancing equipment, (including binoculars, microscope, telescopes, or other such visual magnification) are used to observe the designated celestial events.

The limits of the naked-eye visual acuity is seven on the Stellar Magnitude scale.

The close approach of celestial objects know as a conjunction is probably one of the most frequent of all astronomical events, occurring when two or more celestial objects appear to be aligned within a predetermined circle of close proximity. This circle of close proximity is measured in arc-degrees, which is a measurement of the angle on a specific plane, which plane has a full rotation of 360°. When referring to observations from the Earth, a full rotation completely surrounds the entire planet. Thus, from one Earth horizon to the opposite Earth horizon is 180° and from any horizon to the Celestial North Pole, (Polaris) is 90°.

The actual size of the circle of close proximity, which signifies a close approach during a conjunction, is disputed among most astronomers, but for the sake of simplicity and expediency, I have created my own standard for the size of the circle of close proximity for a conjunction as well as the method to measure it simply without technical equipment.

All celestial object move across the sky in an arc and all movement of these objects occurs at a constant rate of 15 arc-degrees (15°) per hour. Thus, 15° X 24 hours equal 360°. Furthermore, this 15 arc-degrees can be measured with you hand and is a measurement that I have used since I was a young boy during my years as part of the Boy Scouts.

First, hold your right hand to the sky at arms length (with no bend whatsoever in your elbow.) Next bend you wrist so that your hand is at a right angle to your arm. Then hold your hand up against the sky. The distance covered by your four fingers (not including your thumb) is fifteen degrees (15°), or one hour of travel by celestial objects on the celestial sphere. This measurement of four fingers equalling to 15° can be proven by measuring the sun as it climbs or drops near the horizon, while looking at a clock to determine one, two or three hours of travel from the suntouch. ¹

Then extend two fingers and endeavor to cover the two objects of the conjunction behind the width of those two fingers, which two fingers are 7.5° in distance.

Next, try to see if you can cover the same two objects with one of your fingers, and then rotate your finger ninety degrees. If one finger cover all the objects completely during the rotation, then they are in within the 4° conjunction diameter. Too, I use my thumb as a 5 arc-degree circle (5°) which is what I use as the diameter or the circle of close proximity.

Again, I have to say, that this is an astronomy tool that I learned when I began to study the stars during my several years in the Boy Scouts during the 1960s, and one that I continue to use even to this day.

There are several different types of Close Approach, of which is determined by the circumstances of the celestial objects involved. Again, it must be said that all are apparent phenomenon which apparent close proximity is only caused by the viewers perspective.

The first is a Conjunction, which is when two or more celestial objects come to within a predetermined circle of close proximity.

An occultation is the term used in astronomy to refer to any situation in which one object that is in the foreground blocks (occults) a second object in the background.

Occultation events come in several types, Total Occultation, Partial Occultation and those that cast a shadow on a celestial body.

Too, remember, the viewers perspective is always from a location on the Earth and the view is from looking upward to see the objects as they would appear to be on the celestial sphere.

This celestial event occurs when the object in the foreground complete blocks from view the object in the back ground.

This celestial event is a partial occultation that occurs when the object in the foreground does not completely block from view the object in the back ground. This is caused when the foreground object is not large enough to block all of the object in the background.

This celestial event is an occultation that occurs when the object in the foreground blocks the object in the background (either fully or partially) and the object in the foreground casts a shadow on to the observer. This celestial event, of course is an eclipse.