The marked body is tonight's Moon. The fine dashed line is the Moon's path for the month around tonight; the other is the ecliptic, the Sun's path. Each month the Moon passes the Sun, at new Moon, and stands opposite it, at full. If the two lines were one, every new Moon would eclipse the Sun.
They are not one line. The Moon's orbit is tilted 5.145° to Earth's, so for half of each month the Moon runs north of the ecliptic and for half south of it — up to ten of its own widths away, at half a degree each. So a new Moon usually slips past above or below the Sun, and a full Moon above or below Earth's shadow.
The two lines cross twice, at the marked crosses: the nodes. At the ascending node the Moon climbs north over the ecliptic; at the descending node, half a circle round, it drops back south. Only at a node is the Moon on the Sun's line. If a cross is off screen, drag along the dashed path to find it.
Now run a month, a night at a time, with the camera on the Moon. Watch the gap between it and the ecliptic open to five degrees, close to nothing at one node, and open the other way until the next. New and full Moon fall wherever the month puts them, and in most months neither lands on a node.
The same month as a graph: the Moon each night, from two weeks ago to two weeks on, by its distance north or south of the ecliptic. The ecliptic is the zero line and the nodes are where the wave crosses it. New and full Moons land wherever the month puts them, mostly well off the line.
Here is the Sun, on the ecliptic as always — drawn through the ground if it has set. A new Moon happens where the Sun is, so for a solar eclipse it is the Sun that has to be near a node. Thirty degrees from one, the new Moon misses the Sun by 2.6°, five Sun-widths: sin 5.145° × sin 30° is sin 2.6°.
The ring is 17° round the Sun. With a node inside it, a new Moon passes close enough to eclipse the Sun somewhere on Earth. The Sun moves about 1° a day along the ecliptic, so it takes some 33 days to cross the ring — longer than the 29.5 days between new Moons. That window is an eclipse season, and each one brings at least one solar eclipse.
The Sun passes each node once a year, so seasons come in pairs. Not quite half a year apart: the nodes slide west along the ecliptic, 19.3° a year, once round in 18.6 years. The Sun, moving east, meets each again after 346.62 days. So seasons are 173.3 days apart, and come about 19 days earlier each year.
In 2027 the Sun reaches the nodes in February and August, and NASA's catalogue has an annular eclipse of the Sun on 6 February 2027 and a total one on 2 August: six new Moons, 177 days, apart. To tell whether a season is coming, find the Sun on the ecliptic and see how far it is to the nearest node.
Three years of the Sun's distance from the nearer node. Each dip below 17°, twice a year, is an eclipse season; the new Moons inside it are eclipses of the Sun.