First, I know that modeling orbital mechanics of 8 planets is hard, but there are some theories out there, for example, Jupiter is thought to have moved in towards the sun then started moving away. Article

and Uranus and Neptune may have switched spots Article

Is there any pretty good evidence on how the Earth's orbit has changed over time. I remember reading some geological evidence that a year used to be longer, implying that the Earth used to be farther form the Sun, but I've since been unable to find that article, and for purposes of this question, lets count a day as 24 hours even though a day used to be quite a bit shorter hundreds of millions or billions of years ago. - footnote, I've still not been able to find that article but it occurs to me, it could have been counting shorter days, not longer years - so, take that part with a grain of salt.

Are there any good studies out there on how many 24 hour days were in a year, 100, 300, 500, 800 million years ago? or 1 or 2 billion years ago? Either geological or orbital modeling? Preferably something a layman can read, not something written by and for PHDs?

Or any good summaries, also encouraged. Thanks.

I also found this article, but it seems more theoretical than evidence based. http://www.futurity.org/did-orbit-mishap-save-earth-from-freezing/

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    $\begingroup$ A comment to clarify: I believe the usually quoted geological evidence is that the day length has increased, i.e. slower rotation of the Earth caused by tidal effects. (This evidence comes from daily growth marks in fossilised corals - there were around 400 days per year in the Devonian period.) So that particular part doesn't relate to Earth orbit changes. $\endgroup$
    – Andy
    Commented Jul 13, 2015 at 13:50

3 Answers 3


What governs the Earth's orbital period is its orbital angular momentum and the mass of the Sun. Two events have certainly changed the Earth's orbital period (a) whatever collision formed the Moon and (b) the continuous process of mass loss from the Sun. Further possibilities are: (c) Tidal torques from the Sun have increased the angular momentum of the Earth; (d) Radiation pressure from the Sun changes the orbit; (e) drag from the interplanetary medium slows the Earth.

Of these I think only (a) (in the early lifetime of the Earth) and (b) (again, mainly in the first hundred million years or so, but with some effect afterwards) are important.

Early Collisions

Given that (a) probably happened sometime in the first tens of millions of years and likely did not alter the Earth's angular momentum greatly - it depends on the speed, mass and direction of the impactor and the amount of mass lost from the Earth-Moon system - I will ignore it.

Solar Mass Loss

It seems, from observations of younger solar analogues, that the mass loss from the early Sun was much greater than the modest rate at which it loses mass now via the solar wind (or by solar radiation). A review by Guedel (2007) suggests a mass loss rate over the last 4.5 billion years that increases as $t^{-2.3}$ (with considerable uncertainty on the power law index), where $t$ is time since birth, and suggests an initial solar mass between 1% and 7% larger than it is now.

Conservation of angular momentum a means that $a \propto M^{-1}$ and Kepler's third law leads to $P \propto M^{-2}$. Therefore the Earth's orbital period was 2-14% shorter in the distant past due to solar mass loss, but has been close to its current value for the last few billion years.

If the solar wind power law time dependence is very steep, then most of the mass loss occurred early, but the total mass loss would have been greater. On the other hand, a lower total mass loss implies a shallower mass loss and the earth spending a longer time in a smaller orbit.

Tidal Torques

The tidal torque exerted by the Sun on the Earth-Sun orbit increases the orbital separation, because the Sun's rotation period is shorter than the Earth's orbital period. The Sun's tidal "bulge" induced by the Earth applies a torque that increases the orbital angular momentum, much like the effect of the Earth on the Moon.

Quantifying this is difficult. The tidal torque on a planet from a the Sun is $$ T = \frac{3}{2} \frac{k_E}{Q} \frac{GM_{\odot}^{2} R_{E}^{5}}{a^6},$$ where $R_E$ is an Earth radius and $k_E/Q$ is the ratio of the tidal Love number and $Q$ a tidal dissipation factor (see Sasaki et al. (2012).

These lecture notes suggest values of $k_E/Q\sim 0.1$ for the Earth and therefore a tidal torque of $4\times 10^{16}$ Nm. Given that the orbital angular momentum of the Earth is $\sim 3\times 10^{40}$ kgm$^2$s$^{-1}$, then the timescale to change the Earth's angular momentum (and therefore $a$ and $P$) is $>10^{16}$ years and thus this effect is negligible.


Further effects that can be ignored are a changing radiation pressure from the Sun and drag from the interplanetary medium.

Radiation Pressure

The outward force on the Earth is roughly the solar flux at the Earth multiplied by $\pi R^2$ and divided by the speed of light. However, since the radiation flux is, like gravity, obeying an inverse square law, this simply reduces the effective strength of the solar gravity. Because the Sun is getting more luminous with time, by about 30% since it's early youth, then this should result in a weaker effective gravity and the Earth's orbit behaving like it orbits a correspondingly less massive star.

This is a 1 part in $10^{16}$ effect so can safely be neglected.

Drag from the Interplanetary Medium

The density at the Earth is of order 5 protons per cubic centimetre (though I can't immediately locate a reference for this much quote figure) and thus a mass density $\rho \simeq 10^{-20}$ kg/m$^{3}$. The drag force is about $0.5 \rho v^2 \pi R^2$, where $v$ is the velocity of Earth's orbit and $\pi R^2$ is the cross-sectional area presented by the Earth. The torque is then this multipled by $a$ and if assumed constant over billions of years would decrease the angular momentum of the Earth's orbit by 1 part in a billion - so this can safely be neglected.

  • $\begingroup$ Thank you. Very nice detailed answer. I didn't see this until today. $\endgroup$
    – userLTK
    Commented Apr 7, 2018 at 9:37

Earth's orbital eccentricity varies over time from being nearly circular (low eccentricity of 0.0034) and mildly elliptical (high eccentricity of 0.058). It takes roughly 100,000 years for Earth to undergo a full cycle. In periods of high eccentricity, radiation exposure on Earth can accordingly fluctuate more wildly between periods of perihelion and aphelion. Those fluctuations are likewise far milder in times of low eccentricity. Currently, the Earth's orbital eccentricity is at about 0.0167, which means its orbit is closer to being at its most circular.


Since the late heavy bombardment, the Earth's orbit couldn't have changed much. After all, life has been here ever since. Most scientists think that the answer to the faint young sun paradox is a thicker CO2 atmosphere and not the Earth being closer in. In earlier solar system history, the Earth may have been "shepherded" in by Jupiter when it migrated closer.


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