<blockquote><font class="small">In reply to:</font><hr /><p>BTW - earth's equator exactly lines up with the plane of the solar system eccliptic twice a year, at the equanoxes- which are when spring and fall begin, about March 21 and September 21.<br /><br />Then again, that is by definition - the other planet's revolution are angled slighly compared with the solar system eccliptic.<p><hr /></p></p></blockquote><br /><br />Actually, the Earth equator always bisects the ecliptic. This is because the ecliptic is defined by the plane of the Earth's orbit, which means the Earth has an orbital inclination of exactly 0 degrees. Other planets are inclined (slightly or severely; Mars is 1.85 degrees but Pluto is a whopping 17.15), so they cross the ecliptic twice each year. These events are <i>not</i> equinoxes (though pure coincidence could leave a planet with an equinox at this time), but are called plane crossings. It's the time when they pass through the imaginary plane described by the ecliptic.<br /><br />An equinox is something different. It is when the northern and southern hemispheres of a planet receive identical amounts of sunlight. If a planet's rotational axis is inclined relative to the plane of its orbit (not the plane of the ecliptic), each pole will see sunlight for only part of the year, experiencing midnight sun in the middle of summer and sunless "days" in the middle of the winter. Earth's arctic and antarctic circles are defined by the lowest latitude which experiences midnight sun.<br /><br />So, if you were the Sun looking at the Earth, the time of the equinox would occur when the plane of the Earth's equator passes through you. (That plane is not aligned with the ecliptic, however. It is always inclined relative to the orbital plane, because the axis is inclined, and the equator's plane is perpendicular to that -- so the equator is inclined by the axial inclination plus ninety degrees.) If you're standing on the Earth, the equinox occurs when the Sun rises due <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em> -- The Tenth Doctor, "Blink"</font></p> </div>