What if Earth spun on its side like Uranus?

Earth spins like a top as it goes around the Sun. It leans a little to one side.

The planet Uranus leans so far that it is lying on its side! What if Earth did that too?

Earth spins around an imaginary line through the North and South Poles, called its axis. That axis is tilted, leaning about 23 and a half degrees. The tilt is what gives us seasons.

One planet is tipped much further. Uranus spins nearly on its side, rolling around the Sun like a ball. What would happen to day, night and seasons if Earth were tipped over the same way? In this lab you can tip Earth to 90 degrees and see.

A planet's obliquity is the angle between its spin axis and the perpendicular to its orbit. Earth's is 23.44°; Uranus's is 97.77° (NASA fact sheets), so it effectively spins on its side.

This lab sets Earth's obliquity to 90° in a model of solar geometry and asks how the Sun's noon height, the length of daylight and the daily sunlight change through the year at a high-latitude place, at the equator and at the poles.

Because Earth leans, summer days are long and winter days are short.

At the North Pole, the Sun stays up for half the year. Then it stays down for half the year.

With today's tilt, a place 60 degrees north of the equator gets about 18 and a half hours of daylight in late June, with the Sun about 53 degrees high at noon. In late December it gets only about 5 and a half hours, and the noon Sun barely clears the horizon at about 7 degrees.

That is a big swing, but the Sun still rises and sets every day there. Only near the poles does it stay up or stay down for months: because of Earth's tilt, the North Pole has six months of darkness every year.

In the model at 60°N with ε = 23.44°: day 172 gives a noon solar elevation of 53.4°, 18.5 h of daylight and a 24-hour mean top-of-atmosphere insolation of 492 W/m²; day 355 gives 6.6°, 5.5 h and 24 W/m². No day has continuous daylight or darkness at this latitude.

At the pole the model gives 182 days with the Sun up and 182 with it down, consistent with NSIDC's description of the North Pole's six months of darkness each year.

In our pretend lab, Olly tips Earth all the way over.

Now, in summer, the Sun never sets for months! In winter, the Sun never rises for months. Brr!

Choose 'On its side' to set the tilt to 90 degrees, and stay at 60 degrees north. In late June the Sun never sets: 24 hours of daylight, with the Sun about 61 degrees high at noon, and more than twice as much sunlight in a day as today. In late December it never rises at all.

Step through the year with the day slider. In the model, this place gets about 122 days in a row of non-stop daylight and about 121 days of darkness. Even the equator changes: at the solstices, the Sun just circles low around the horizon all day.

With ε = 90° at 60°N, the model gives continuous daylight from about day 111 to day 232 (122 days) and polar night for 121 days. On day 172, noon elevation is 60.7° and daily insolation 1,179 W/m², 2.4 times today's; the noon Sun reaches almost 90° at its yearly peak.

At the equator, insolation on day 172 falls to 5 W/m² (noon elevation 0.7°) against 433 W/m² at the equinoxes. Averaged over the year, the model's pole receives 434 W/m² and the equator 276: above about 54° of tilt, the poles out-sun the equator.

The tipped Earth's pole points at the Sun for part of the year. Then it points away.

When your part of Earth points at the Sun, it is always daytime. When it points away, it is always night.

Earth's axis keeps pointing the same way in space as Earth travels around the Sun. With a small tilt, the poles only lean a little toward and away from the Sun. With a 90-degree tilt, the axis lies flat, so at one point in the orbit the North Pole points straight at the Sun, and half a year later straight away from it.

When your half of the planet faces the Sun the whole time it spins, the Sun never sets. When it faces away, the Sun never rises.

The solar declination is δ = asin(sin ε · sin λ), where λ is the Sun's ecliptic longitude. With ε = 23.44° the subsolar point stays within the tropics and only latitudes within about 23° of the poles get polar day and night (OpenStax); with ε = 90° it sweeps from pole to pole, so every latitude except the equator has some polar day and polar night.

The model's limits, from its documentation: a circular orbit, local solar time, a flat horizon without refraction, and sunlight measured at the top of the atmosphere. It does not model climate: what weather, ice or temperatures such a planet would have is an idea, not a result.

Uranus really is tipped over on its side. It rolls around the Sun like a ball.

Its year is very long, so each winter lasts many, many years.

Uranus is the only planet whose equator is nearly at a right angle to its orbit, tilted 97.77 degrees. NASA says this may be the result of a crash with an Earth-sized object long ago. It gives Uranus the most extreme seasons in the solar system.

Uranus takes about 84 Earth years to go around the Sun. For nearly a quarter of each Uranus year, the Sun shines almost directly over one pole, and the other half of the planet has a dark winter lasting 21 years.

NASA gives Uranus an obliquity of 97.77° and an orbital period of about 84 Earth years (30,685 days), and notes that the tilt may result from a collision with an Earth-sized body. For nearly a quarter of each orbit the Sun is nearly over each pole in turn, leaving the opposite hemisphere in a 21-year winter.

An Earth tipped to 90° would share the geometry but not the timescale: its polar days and nights would last months, not decades, because Earth's year is 84 times shorter.