Minute and second of arc arcminutes and arcseconds to measure angles when the object being measured is too small for direct visual inspection. For instance, a toolmaker's optical comparator will often include an option to measure in "minutes and seconds". Minute and second of arc A minute of arc, arcminute (arcmin), arc minute, or minute arc is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn – it is for this reason that the Earth's circumference is almost exactly 21,600 nautical miles. A minute of arc
Mean longitude angle between any physical objects. It is simply a convenient uniform measure of how far around its orbit a body has progressed since passing the reference direction. While mean longitude measures a mean position and assumes constant speed, true longitude measures the actual longitude and assumes the body has moved with its actual speed, which varies around its elliptical orbit. The difference between the two is known as the equation of the center. From the above definitions, define the "longitude of the pericenter" Then mean longitude is also Another form often seen is the "mean longitude at epoch", "ε". This