text: I would say an isotherm is a line connecting points having the same temperature.\n\nAnd the heat equator is the great circle lying just underneath the sun in a given short time period. To keep it thin, we'll pick one day and say the sun's rays are vertical at all points of the circle when it is noon at each of those points.\n\nThis equator would thus divide the earth into two equally heated halves over the given time period.\n\nDifference:  Temperatures not necessarily uniform along the heat equator, but are the same along an isotherm.\n\nBut I have not been through college, nor been trained with respect to these definitions, so I might be wrong.\n\nSometimes things can be arbitrary rather than appealing to some logical sense; however, that is less frequent in scientific disciplines.