how often are the stars in the same alinement??

Question

ie at this date&time the plants where at the place than agine on this day&same time but years later.

Answer 1

Maybe you mean planets. This is what I calculate below.


The average time period between the passage of a planet through the same Earth-Sun line is called the synodic period.

For Mercury, the synodic period is 115.88 days. Every 115.88 days, Earth, Mercury (and the Sun) are in line.

Venus has a synodic period of 583.92 days. The smallest common multiplier of 115.88 and 583.92 is 16916.1624 days.

We'll drop the last two (and keep a precision of 2 decimal places).

So, if on a day, Mercury Venus and Earth are on the same line (relative to the Sun), then they will be in line again 16,916.16 days later.

In practice, because the position of perihelion and aphelion are not the same on all orbits, the alignment will not be as perfect the second time as the first. But, for now, we'll ignore that. Like the ancients, we'll pretend that the orbits are circular and orbital speed is constant.

Mars = 779.88
The next 'lowest common multiple' (at 2 decimal accuracy) is 1099381.39 days (very close to 3,010 years).

Jupiter = 398.89 days
This number is relatively prime (using 2 decimal places) to the cumulative synodic period we have so far, so it simply gets multiplied to the 3,010 years.

If you start with all planets (Mercury to Jupiter) in line, then you'd have to wait
43,870,477 days (more than 120,000 years)
(From this point, we'll use integers only; remember that for a true alignment, you may have to wait longer than these numbers suggest)

Saturn 378.08 (we'll use 378)
378 and 43,870,477 only share one factor (7) so we must multiply them, then divide by 7:
2,369,005,758 days (almost 6.5 million years)
=2*3*3*3*7*1033*6067


Uranus = 369 (= 3*3*41)
We already cover the 3*3 portion, so we only need to multiply by 41.
The cumulative synodic period (Mercury to Uranus) is
2*3*3*3*7*41*1033*6067 days (over 265 million years).

Neptune 367.47 (let's use 368 because 367 is prime)
368 = 2*2*2*2*23
We have one '2' covered we need to use three of them along with the 23.

2*2*2*23 = 8*23 = 184
184 times 265 million years = almost 49 billion years
(which is about ten times the present age of the solar system).

So, if you begin with all 8 planets (plus the Sun) on the same line, you'd have to wait longer than the expected lifetime of the solar system to get them all on the same line again.

This is for a perfect alignment.

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However, you may be tolerant and prefer to call an alignment any time that the planets are within xx degrees of a line. Depending on how large you make the angle xx, the time period could be much shorter.

For example, the "Great Alignment" that was used for the slingshot effect sending the Voyager probes on a Grand Tour of the outer solar system used and angle of xx = 90 degrees.

In other words, four outer planets (plus Pluto) were in the same 'sector' (within 45 degrees of the Earth-Sun line) and that was rare enough to be called an alignment.

This kind of alignment will occur 176 years later (1970 - 2146)

Answer 2

Think about your question and ask it again. At present it is gibberish. Be a little more careful with nomenclature, stars are not planets, if you are going to ask technical questions about astronomy, it is best to know the difference.

Be a little more careful with spelling, use the spell checker if you have to, but do not trust it. Astronomers tend to be highly educated and most of them can spell fairly well, so they can spot an idiot at 100 paces.