FYI: This question goes beyond eclipse basics. We all hear the same rhetoric that a solar eclipse occurs when the sun, moon, and earth are aligned. Simple, right? Not when one tries to quantify things.
The basic question is this: The sun and moon remain within the zodiacal band all year. This band corresponds to points on earth known as the tropics: a band roughly between -23 degrees and +23 degrees latitude. So, if the sun and moon align, how can the shadow fall outside the tropics? If the sunlight is normal to the earth’s surface over the tropics, shouldn’t the moon’s shadow fall normal to the earth’s surface, too? Yet, solar eclipses can be observed all over the globe. I am having a hard time rectifying this discrepancy.
The advanced question is: When one assigns RA and Dec coordinates to the point of totality, assuming the sun and moon are two of three co-linear points, the third co-linear point on earth would have to be in the tropics, wouldn’t it?
Ultimately, if the sun and moon share the same point in the sky, what other factor(s) determine whether the moon’s shadow hits at +45 latitude or +32 latitude or +26 latitude? Am I overthinking this?
I think your mistake is assuming that the alignment has to be technically perfect–that is, that if you continue the line from the sun to the moon, is must pass through the center of the earth. If that alignment is less than perfect, though, you might get a “grazing eclipse” where the totality just sweeps over the south poole, or the north pole, or anywhere in between.
Put another way; the zodiacal band is in the sky, and also on the earth, which are the places on the earth where the zodiacal band in the sky is directly overhead. But you can still see the sun in the sky when it is not directly overhead, and the same for the moon. You might even imagine a “sunrise eclipse”, where the sun and moon rise over the horizon at the same time and place, and you get a moment of totality before the eclipse slips out of alignment.
Do either of these help?
Edited to add: The moon is not going to be at exactly the same point in the sky for observers at different parts of the earth, because of parallax. (Same for the sun, though the variance there is much slighter.) The moon’s shadow “hits” at the point on earth where the correspondence is closest to perfect.
You’re assuming the three center points all must aline exactly … and that’s not the case … if the Moon is a little above the Earth-Sun line then the path of totality will run a higher than the equator, perhaps even missing the Earth all-together … in the upcoming eclipse in the USA, the Moon is only a little higher than the Earth-Sun line and the shadow will only be a little above the equator …
The path of the Sun across the background stars is mostly constant year-after-year … however the Moon’s orbit is a little inclined to the path … solar eclipses only occur when the Moon is crossing the sun’s path, and this is twice a year … if the Moon crosses the exact plane at exactly new moon, then we’ll have an eclipse along the equator … any variance and the eclipse will be seen elsewhere, including the poles …
So, the answer to your advanced question is that the three points need not be exactly co-linear … they need to be close, but not exact …
Please use #12 welding glass or better to watch the partial phases … for the most part … only actual totality can be observed without eye protection … safety first
Here’s an activity for you: Draw a small circle representing the Earth. Draw a short, wide X through that circle, 47 degrees wide, for the tropic band. Put a dot close to the Earth, somewhere in that band, for the Moon, and then put a dot further away, also in that band, for the Sun. Now, draw a line between the Sun and the Moon. Most of the time, that line will miss the Earth completely: Hence why we don’t have an eclipse every month. And even when it hits, it’ll often hit only the edge of the Earth. Where that line hits the Earth certainly does not need to be within the tropic band, and in fact it usually isn’t.
Or to put it another way, the three points must be co-linear, but they are not necessarily the exact centers of the bodies in question. One of the three points, in fact, should be the eye of the observer (human or mechanical) on the surface of Earth.
An even simpler way to think about it is this. As the Moon orbits the earth, it passes “between” the Earth and Sun once a month. Most months, the shadow it casts misses the earth completely and there is not even a partial eclipse of the Sun. About every 18 months on average (once every 18 or 19 orbits*), the passage is close enough to being in a straight line so that there is an eclipse seen some place on Earth. The way you’re thinking there would be an eclipse every month.
*Those are synodic orbits (with respect to the sun).
“Within the zodiacal band” is only very approximate. The plane of the Moon’s orbit is tilted about 5° wrt the ecliptic (the perceived plane of the Sun around the Earth).
That 5° may not seem like much but on its own it can account for a difference of 43k miles how high or low the shadow of the Moon can go near Earth, which is only 8k miles in diameter.
Actually it’s more like 4 eclipses every year, two solar and two lunar, of some type … they all don’t have to be total …
You know this guy knows what he’s talking about since he posted this five days from now !!!
Fred Espenak from NASA compiled the landmark catalogue of eclipses which is probably the single best resource for dates, times and parameters for those that occur between 2000 BC and 3000 AD … NASA Eclipse Web Site … enjoy …
There was an annular eclipse some years ago (I don’t remember the year) and it appeared almost exactly at sundown, here in San Diego. A “McDonalds” golden arch!
ftg has it right. The moon varies from 5 deg above the plane of the ecliptic to 5 deg below. It is only when it is (nearly) on the ecliptic that you get an eclipse. That is why you often have a solar and lunar eclipse within 15 days of each other.
Eclipses always happen at the lunar nodes … those two point along it’s orbit where the Moon crosses the Earth’s orbit around the Sun … that happens a little less than every six months … and we’ll always have some manner of solar and lunar eclipse two weeks apart …
That is, the (almost) circle that the Moon traces out around the Earth is tilted with respect to the (almost) circle that the Earth traces out around the sun. These two planes meet on a line that basically goes through the center of the Earth, and where this line meets the Moon’s orbit, that is a node. They are often called “ascending” and “descending” nodes due to northern hemisphere hegemony, and the “ascending node” is more properly something like “north-heading node”, when the Moon passes from the South of the Earth-Sun disk to the North.
If the Moon passes in between the Earth and Sun (or Earth between the Moon and Sun) when the Moon close to the nodes, then the three bodies can line up such that the shadows fall on the other. At times when the Moon passes between the Sun and Earth that is not at one of these nodes, then the shadow of the Moon is too far to the north or south of the Earth.
