Analemma Explained: How To Make It Easily
Getting those stunning pictures of the Sun doing strange curves in the sky isn't a quick job — it takes a whole year! In this article, we'll tell you how these curves work and how you can make your own in just a few taps.
Contents
- What is an analemma?
- How to make your own analemma
- What is the meaning of analemma?
- Why is the analemma a figure eight?
- What did analemma look like in the past?
- What does analemma look like on other planets?
- Bottom line
What is an analemma?
An analemma is the closed curve resembling an elongated figure eight that the Sun makes in the sky over a year if you capture its position at the same time every day from a fixed spot. In astronomy, the term "analemma" is used in reference to the Sun, but the Moon's figure-eight curves are also commonly called analemmas, even though they're made a bit differently.
Making a picture of an analemma is known as one of the most difficult and time-consuming things in astrophotography! You need to set up a camera in one spot and snap the Sun multiple times throughout the year, always at the exact same time of day. Then you should overlay all these pictures together and get the final result. By the way, analemmas’ appearance depends on multiple factors that we’ll discuss further.
Analemma from the Northern Hemisphere vs. Southern Hemisphere
Let's talk about what the analemma looks like in different places on the Earth; suppose we make an analemma around solar noon when the Sun is at its highest point in the sky.
Analemmas captured in the Northern Hemisphere appear as a figure eight with a smaller loop on the top. If we were to capture an analemma up at the North Pole, we would see only the top half of an upright figure eight.
In the Southern Hemisphere, the smaller loop is under the bigger one. At the South Pole, only the upper half (the bigger loop) would be visible above the horizon.
Remember that the Sun is highest on the analemma in summer (June in the Northern Hemisphere, December in the Southern Hemisphere) and lowest in winter. The times in between make up the rest of the pattern. To capture an upright standing figure eight, create an analemma near solar noon. To capture a tilted analemma, choose the time before or after the Sun reaches solar noon.
How to make your own analemma
There are two ways to make an analemma on your own: the hard way and the easy way. We're sticking to the easy route here, but if you're feeling adventurous, you can find all the tricky steps on Instructables.com. Here's how you can build an analemma for any location in just a minute:
- Open the Sky Tonight free app (download it here if you don't have it yet);
- Spot the Sun on the sky map and tap it;
- Hit the tiny camera icon that appears at the bottom;
- Voila, you've got a solar analemma!
- Overlay the analemma on the image from your camera by tapping the big blue round icon two times.
You can adjust the analemma’s appearance using the panel at the bottom. The four of five lower icons control the date display, visibility beneath the horizon, pattern density, and line display. The first icon in the row switches between different curve types.
How to make curves for the Moon and other sky objects
The fun part doesn’t end there. The Sky Tonight app lets you create cool curves for other celestial objects, too, like the Moon, planets, dwarf planets (except Pluto), asteroids, and comets!
The lunar figure-eight curves are particularly interesting. They look like the analemma, but to create them, you mark the Moon’s position with a slight delay — every 24 hours and 51 minutes instead of the usual 24 hours. This represents the time it takes for the Moon to return to the same position in the sky. The entire curve covers the duration of a lunar month.
To make curves for other objects, follow the same steps, as for the Sun. You can also change the settings using the icons at the bottom of the unfoldable panel.
What is the meaning of analemma?
There is an apparent solar time, which runs at varying speeds throughout the year because of the way the Earth orbits the Sun. There is also a mean solar time — a consistent time kept by most clocks and watches that would be measured by observation if the Sun traveled at a uniform apparent speed throughout the year. The difference between the apparent solar time and mean solar time is known as the equation of time. An analemma, captured every day at the same mean solar time, is basically a diagram of the equation of time.
The Earth has a slightly elongated orbit, and when it’s closer to the Sun (around January each year), it moves faster due to the gravitational forces. Also, the Earth’s axis is tilted: our planet’s Northern Hemisphere is tilted towards the Sun from March to September, while its Southern Hemisphere is tilted towards the Sun from September to March.
The combined effects of the elliptical orbit and the tilt result in the length of each solar day being slightly different from the previous day. This is why apparent solar time varies throughout the year. For example, in November, apparent solar time could be 16 minutes ahead of mean solar time, in February it is 14 minutes behind, and in April and September, the two times cross over, carrying on in an endless cycle.
Why is the analemma a figure eight?
The analemma appears as a slightly elongated figure eight because of the Earth’s axial tilt and elliptical orbit. Here is how it works.
The basic figure-eight shape of the analemma comes mainly from the tilt of the Earth's axis. Due to the tilt, from the Sun’s perspective, the Earth appears to wobble while orbiting the Sun. And because of this wobbling, as seen from a fixed position on the Earth, the Sun appears to make the figure eight pattern in the Earth’s sky. Without the axial tilt, the analemma would be oval-shaped. At the equator, it would be a straight line from west to east.
The asymmetry of the analemma is due to the Earth's elongated orbit. If the Earth’s orbit was circular, the top and bottom loops of analemma would be identical.
At different times of the year, the Earth moves closer and further away from the Sun. Near the closest point to the Sun (perihelion in January), our planet is speeding up, so the Sun draws a larger loop in our sky. If you look at analemmas captured from different locations, you’ll see that the analemma pattern becomes less dense and wider everywhere around January.
The Earth rotates once on its axis in about 23 hours 56 minutes (this is called a sidereal day). But it takes about 4 more minutes for the Sun to return to the same point in the sky or local meridian (this is called a solar day). If you were to set up a sundial and start a stopwatch at noon one day and stop it at noon the next day, you would measure a solar day.
On two consecutive days, our planet must rotate a little extra on its axis for the Sun to return to the same place. During a single sidereal day, the Earth rotates 360°, and to complete the solar day, it must rotate 1° more (for a total of 361°). And as the Earth’s speed of orbit changes, so does the extra angle the Earth must rotate to bring the Sun back across the meridian.
Near the perihelion, the Earth speeds up and has to rotate a little more. Therefore, the Sun takes longer to move from one solar noon to the next. When our clocks say it's noon, the Sun has not yet reached the local meridian. The opposite happens near the aphelion when the Earth is moving more slowly in its orbit. At this time, the Earth does not have to make as large an angle to bring the Sun back over the local meridian. By the time our clock says noon, the Sun has already crossed the local meridian.
It's also important whether the Earth's perihelion coincides with a solstice or equinox. When the Earth's perihelion coincided with the solstice, as it did 750 years ago, the analemma had a north-south axis of symmetry. Today, the Earth reaches perihelion about 12 days after the winter solstice, so the analemma is slightly tilted.
What did analemma look like in the past?
As we have already mentioned, the appearance of the analemma from the Earth depends on the latitude and the time of the day. But they also looked different in the past and will look different in the future. Let's take a quick look at what exactly has changed over time.
Due to the slow precession of the Earth's axis and other factors, the equinoxes and the Earth's perihelion point gradually shift along the ecliptic. Now, they’re slowly approaching each other. In 1246, perihelion occurred on the date of December solstice; in 6489, it will occur at the March equinox, in 11,732 – at the June solstice, and in 16,974 at the September equinox. The loops of the analemma curves are different in all these cases.
Over the long term, both the eccentricity and the longitude of perihelion undergo significant changes. For example, about 100,000 years ago, the Earth's orbit was much more elliptical than it is today, with an eccentricity of approximately 0.0473. About 98,500 B.C., perihelion coincided with the September equinox, causing the analemma to tilt slightly. About 93,000 B.C., perihelion coincided with the December solstice. Because of this and greater eccentricity, the analemma looked like a drop!
What does analemma look like on other planets?
Analemmas, those shapes formed by the Sun's position in the sky over a year, vary from planet to planet. Each one has its own unique look, and here's a simple breakdown.
Take Mars, for example. Its analemma resembles a teardrop due to a combination of factors like high eccentricity (0.0934 compared to Earth's 0.0167) and its perihelion and aphelion aligning with its equinoxes.
Now, analemma, as seen from Mercury, is very simple. With a small axial tilt (less than one degree), its analemma is just a dot. The analemmas of Venus and Jupiter have an elliptical shape. Saturn? Think teardrop with a small loop. Neptune and Uranus? They showcase a figure-eight pattern as well.
Bottom line
An analemma might seem like just the Sun's apparent path in the sky throughout the year, but it's way more complicated in the world of astronomy. Its shape is influenced by many factors and its explanation involves some complex math. In this article, we've broken down this term in simple language. Plus, we've shared a quick way for anyone to create their own analemma in just a minute. Grab the Sky Tonight app — it's your easy way to understand astronomy!
