When a Day Is Longer Than a Year

Ever have one of those days where seconds seem like hours? The length of this one particular day stretches on and on until it seems as though a year has passed.

There’s a place where this happens literally every day. Before you start making vacation plans, I had better tell you where this place is.

I’m talking about Venus, our next-door neighbor in our solar system. Venus goes around the sun in less time than it spins around on its axis, so a day on Venus is longer than a year, although not by too much. A Venusian day is about 243 Earth days, while a year on Venus is about 225 Earth days.

Imagine Venus being more Earth-like. Instead of pizza-oven temperatures (860 degrees Fahrenheit, or 460 Celsius), we have an average around 59°F (15°C). Instead of an atmosphere 90 times more intense than Earth’s—with enough pressure to crush a typical submarine—we get a stable, boring single atmosphere of pressure. Instead of suffocating on carbon dioxide, imagine us being able to breathe the air and survive.

Let’s keep that day/year relationship going in our little thought experiment, but otherwise imagine everything else to be Earth-like.

Our ancient ancestors kept time with an always-on, incredibly accurate display. This was the sky, and the movement of the key players on this display screen acted like a metronome—a device that could be used to tell time reliably.

When it was time to meet somewhere, the number of sunrises or sunsets (days) was an incredibly useful standard of measurement.

On Venus, this would probably be no different, except for the fact that the planet takes such a wild amount of time to rotate. Remember 243 Earth days for one Venusian day?

Well, this is a little misleading. Unlike Earth, Venus spins in a retrograde direction, meaning it rotates opposite to the direction of its orbit around the Sun. Not only does this mean that the Sun rises in the west and sets in the east, but it also means what we call a “solar day” (the time for the Sun to return to the same place in the sky) is about 117 Earth days.

If you’re wondering why Venus’s day is so long, and why it spins “backwards”, you are not alone. Scientists suggest that Venus’s thick atmosphere has slowed its rotation over time, or that there could have been a massive impact from another planet-sized object, like the one that created Earth’s Moon.

What would life be like with a 117-Earth-Day solar day on Venus? First of all, we’re talking about 58 or 59 days of light, followed by 58 or 59 days of darkness. Basically, the “day” portion lasts almost 2 Earth months, and so does the night.

In the same way our ancient ancestors used the Moon as a way to tell when to plant crops, Venusians might have divided their “day” into two equal portions: sunrise to high noon, and then high noon to sunset. In this way, they might have ended up with an easy, reliable measure of time roughly equal to an Earth month.

Indeed, dividing 58.5 days of light by two gives us 29.25 Earth days for one Venusian “half day.” One lunar month on Earth is just over 29.5 days.

Human sleep cycles are wired for a 24 hour day, but 58.5 days of light changes a lot. Here on Earth, the closer you get to the poles, the midnight sun can last for months, and so can the polar night. The continuous light or darkness disrupts the circadian rhythm, the body's natural sleep-wake cycle.

We humans need sunlight so that we can produce Vitamin D, a necessary ingredient for our bones, muscles, and brains. Melatonin and serotonin, two hormones that play big roles in our sleep cycle and our emotional state, are badly disrupted by polar nights.

Indigenous communities in the Arctic and Antarctic regions have developed cultural practices and traditions over centuries to adapt to the extreme variations in sunlight. I’m confident that our Venusian ancestors would develop similar methods for dealing with this, but I’m also quite certain that we (humans) would have turned out quite differently.

Here, the fantasy is laid bare: we can’t talk in earnest about an agricultural revolution arising in the same manner as on Earth, since a completely different set of staple foods would have to develop around a four-month long day. Completely different species would have evolved on Earth-Venus, and human beings probably wouldn’t even be on the menu.

The fantasy may be ruined, but the thought experiment is not. Help me think through what a Venusian day would mean for rituals, the economy of Venus, and for social ties. What unique conclusions would the Venutians draw about the universe that might be different from ours?