# Why is every year the same number of days despite the gravity in the solar system?

I was just watching a Geoff Marcy talk on YouTube showing how they infer the presence of planets transiting distant stars. The supposed periodicity wasn't always quite regular from what I could see, and so I wondered if there might be something tugging at the planets as they orbit their star. Then I thought of the earth: Surely there are some years when we are aligned with Jupiter so much so that it would exert some sort of perceptible influence on the planet, thus lengthening the year, even if only by some hours or days, considering that the sun is much more massive. And what would happen every few thousand (?) million (?) years when all the planets and even some of the larger asteroids, Kuiper Belt Objects - everything but the kitchen sink - come into alignment? Wouldn't the earth be drawn outwards to some extent, thus making a year perhaps 370 days long (or whatever)? And maybe temperatures would drop with the planet a fraction of an AU farther from the sun? I know that this is a website for the already enlightened, but I would appreciate an answer that even Josephine Bloggs would understand.

Derived from this paper the interval between March equinoxes for 1989/1990 has been 4 minutes 28 seconds longer than the corresponding interval for 1987/1988.

So the length of a year can vary several minutes, e.g. by gravitational effects of other planets. For calendars a mean year is used. Atomic clocks run far more precise than the more or less periodic motion of Earth.

All planets aligned don't cause anything grave. A similar kind of sysygy has been 1982.

• Is this not because of the equinox precession? Apr 1 '14 at 3:22
• @Py-ser Precession adds a 20 minutes 24.5 seconds difference between sidereal and tropical year: en.wikipedia.org/wiki/Sidereal_year. To make things even more complicated there is also a nutation: en.wikipedia.org/wiki/Nutation Apr 1 '14 at 11:19

Surely there are some years when we are aligned with Jupiter so much so that it would exert some sort of perceptible influence on the planet, thus lengthening the year, even if only by some hours or days, considering that the sun is much more massive.

Let's do the math. The force of gravity is $$F = \frac{Gm_1m_2}{r^2}$$. When in alignment, the Earth and Jupiter are about 600 million km apart ($$r$$). Jupiter masses about $$2x10^{27}$$kg ($$m_1$$) and the Earth about $$6x10^{24}$$ kg (m_2). That gives about $$2x10^{18}$$N of force. This is a lot, but is it a lot compared to the Sun?

Now let's do the same for the Sun and Earth. They're about 150 million km apart and the Sun masses about $$2x10^{30}$$kg. The force is about $$3.5x10^{22}$$N or 10,000 times more.

Jupiter has an effect on the Earth's orbit, but at its strongest point the effect is 10,000 times smaller than the Sun. It has a small but noticeable effect on the Earth's orbit.

Gas giants do have a dramatic effect on the Solar System, but over the course of very, very long time scales. Jupiter and Saturn may have prevented a planet from forming in the Asteroid Belt and may explain why Mars is so small compared to Earth and Venus. Jupiter Trojans are groups of asteroids locked into orbit 60 degrees in front of and behind Jupiter. The point around which Jupiter and the Sun rotate lies just outside of the Sun making the Sun wobble.

And what would happen every few thousand (?) million (?) years when all the planets and even some of the larger asteroids, Kuiper Belt Objects - everything but the kitchen sink - come into alignment?

Compared to Jupiter and the Sun, these are peanuts.

The Sun contains about 99.9% of the mass of the Solar System. Of that 0.1% left over Jupiter contains about 70%. Saturn, Uranus and Neptune make up about 29%. Everything else is about 1%.

The force of gravity falls off as the square of the distance; at twice the distance gravity is four times weaker, at four times the distance its sixteen times weaker. As things get further away from the Earth their gravity gets much, much weaker. Saturn's effect on the Earth is about a tenth of Jupiter's. Uranus, smaller and further away, is even weaker. Neptune even weaker.