Tag: Milankovitch cycles

More Perfect Than Earth

Did you know that both Venus and Neptune have orbits around the Sun that are more circular than the orbit of the Earth? In fact, Venus has the most circular orbit of all the major planets in the solar system. Here are the current orbital eccentricities of these three planets.

Venus0.007
Neptune0.010
Earth0.017

Perhaps surprisingly, there are currently 1,959 minor planets with orbits more circular than that of Venus. There may be more, but I’ve excluded any minor planets where the 1σ uncertainty in eccentricity is greater than the eccentricity itself. Large uncertainties happen because some minor planets do not yet have well-determined orbits.

To ensure that my restriction of σe < e wouldn’t be too severe, I confirmed that minor planet 232812 has the lowest known orbital eccentricity. The 11 minor planets that nominally have a lower eccentricity are all unnumbered, which means their orbits are not well determined and should thus be excluded.

The most circular orbit of all minor planets with a well-determined orbit is 232812 (2004 RG298). It has not yet received a name. It is a main-belt asteroid and has an amazingly circular eccentricity (e) of 0.0000152412. It orbits at a distance of 2.37 AU from the Sun once every 3.65 years. Its orbit is inclined 2.4° to the ecliptic. Its physical size is unknown

Among the named minor planets, the one with the most circular orbit is 12083 Darone. It is also a main-belt asteroid and has an eccentricity of 0.00177265. It orbits at a distance of 2.13 AU from the Sun once every 3.11 years. Its orbit is inclined 2.6° to the ecliptic. 12083 Darone has a diameter between 3.5 and 3.8 km. The lucky person to have this asteroid named after them is Gregory Darone, who mentored a finalist in the 2019 Regeneron Science Talent Search, a science competition for high school seniors. He teaches chemistry at the Charter School of Wilmington, Wilmington, Delaware.

The two largest trans-Neptunian objects are Pluto and Eris—both with high orbital eccentricities—and among the major planets, Mercury has the highest orbital eccentricity. Here are their current values.

Eris0.438
Pluto0.245
Mercury0.206

Eris has the highest orbital eccentricity of these three objects by far. Eris orbits between 38.2 and 97.7 AU from the Sun. Its current distance is 95.5 AU and it is very gradually getting closer to the Sun. At times during their respective orbits, Eris can be closer to the Sun than Pluto. Eris orbits the Sun once every 559.9 years at an orbital inclination of 43.9° with respect to the ecliptic.

Pluto orbits between 29.6 and 48.7 AU from the Sun. Its current distance is 35.6 AU and it is gradually getting further from the Sun. During the years 1979 to 1999, Pluto was closer to the Sun that Neptune, and this will next happen again from 2223 to 2243 (approximately). Pluto orbits the Sun once every 244.7 years at an orbital inclination of 17.0° with respect to the ecliptic.

Mercury orbits between 0.31 and 0.47 AU from the Sun. Mercury orbits the Sun once every 88 days at an orbital inclination of 7.0° with respect to the ecliptic.

All three of these objects are smaller than Jupiter’s moon Ganymede!

Incidentally, there is no known minor planet that orbits completely inside the orbit of Mercury, but (as of this writing) 523 asteroids and 1,608 comets have come closer to the Sun than Mercury ever does. Sometimes a comet comes so close to the Sun that it disintegrates completely. Actual collisions with the Sun do occur.

Getting back to our original topic of nearly-circular orbits, the Earth’s orbital eccentricity has ranged from as circular as 0.000055 to as elliptical as 0.0679 due to gravitational perturbations caused by all the other objects in our solar system. This variation in orbital eccentricity is one of the Milankovitch cycles. Currently, the Earth’s orbital eccentricity is becoming more circular.

The orbits of all objects in our solar system change with time (albeit usually gradually and by a gentle amount). Over millions or billions of years, the gravitational attraction from stars passing close to the solar system can nudge orbits, too. Our solar system is a much more dynamic place than anyone imagined a century ago. Barring the extremely unlikely event of a star (lots of advance warning) or a rogue black hole (much less advance warning) entering our solar system, the Earth’s orbital eccentricity should remain stable for a very long time to come.

Milutin Milanković

Serbian engineer, mathematician, and scientist Milutin Milanković was born 140 years ago on this date in 1879, in the village of Dalj on the border between Croatia and Serbia—then part of the empire of Austria-Hungary. He died in 1958 in Beograd (Belgrade), then in Yugoslavia and today in Serbia, at the age of 79.

Milanković is perhaps most famous for developing a mathematical theory of climate based on changes in the Earth’s orbit and axial orientation. There are three basic parameters that change with time—now known as the Milankovitch cycles—that affect the amount of solar energy the Earth receives and how it is distributed upon the Earth.

I. Orbital eccentricity of the Earth changes with time

The eccentricity (e) tells you how elliptical an orbit is. An eccentricity of 0.000 means the orbit is perfectly circular. A typical comet’s orbit, on the other hand, is very elongated, with an eccentricity of 0.999 not at all uncommon. Right now, the Earth’s orbital eccentricity is 0.017, which means that it is 1.7% closer to the Sun at perihelion than its semimajor axis distance (a), and 1.7% further from the Sun at aphelion than its semimajor axis distance.

The greater the eccentricity the greater the variation in the amount of solar radiation the Earth receives throughout the year. Over a period of roughly 100,000 years, the Earth’s orbital eccentricity changes from close to circular (e = 0.000055) to about e = 0.0679 and back to circular again. At present, the Earth’s orbital eccentricity is 0.017 and decreasing. We now know the Earth’s orbital eccentricity changes with periods of 413,000, 95,000, and 125,000 years, making for a slightly more complicated variation than a simple sinusoid, as shown below.

II. Tilt of the Earth’s axis changes with time

The tilt of the Earth’s polar axis with respect to the plane of the Earth’s orbit around the Sun—called the obliquity to the ecliptic—changes with time. The Earth’s current axial tilt is 23.4°, but it ranges between about 22.1° and 24.5° over a period of about 41,000 years. Greater axial tilt means winter and summer become more extreme. Presently, the axial tilt is decreasing, and will reach a minimum around 11,800 A.D.

III. Orientation of the Earth’s axis changes with time

The Earth’s axis precesses or “wobbles” with a period of around 26,000 years about the north and south ecliptic poles. This changes what latitude of the Earth is most directly facing the Sun when the Earth is closest to the Sun each year. Currently, the southern hemisphere has summer when the Earth is at perihelion.

Milanković used these three cycles to predict climate change. His ideas were largely ignored until 1976, when a paper by James Hays, John Imbrie, and Nicholas Shackleton in the journal Science showed that Milanković’s mathematical model of climate change was able to predict major changes in climate that have occurred during the past 450,000 years.

These Milankovitch cycles are important to our understanding of climate change over much longer periods than the climate change currently being induced by human activity. Note the extremely rapid increase of greenhouse gas concentrations (CO2, CH4, and N2O) in our atmosphere over the past few decades in the graphs below.

https://www.noaa.gov/news-release/greenhouse-gases-continued-to-increase-rapidly-in-2022

The world population has increased by 93% since 1975. In 1975, it was about 4 billion and by 2020 it is expected to be 7.8 billion.