Unlike the Moon, which has a permanently unseen “far side,” the Sun is a rotating sphere of plasma. The Sun completes one full rotation in approximately 25 Earth days at its equator, meaning the hemisphere facing away from us is constantly changing. This “other side” is simply the portion of the Sun currently obscured from Earth’s perspective, rotating into and out of sight over about two weeks.
Defining the Hidden View
The limitation in observing the Sun’s entire surface is purely a matter of geometry, or line-of-sight blockage. As the Sun spins on its axis, any feature on its surface, such as a large sunspot group, rotates out of our view and spends about 14 days on the far side. Because Earth orbits the Sun in the same direction, the time it takes for a feature to return to the same apparent position, known as the synodic rotation period, is slightly longer, around 27 days. This two-week period of invisibility is what scientists have worked to overcome.
The constant rotation means there is no fixed, hidden area of the Sun that perpetually remains averted from Earth. The challenge for solar physicists is that an active region can develop, grow, and release powerful eruptions while entirely out of sight. These phenomena can then rotate into a direct line-of-sight with Earth with little to no prior warning. This potential for unexpected space weather events drove the development of specialized observation techniques.
The Search for Unseen Objects
The idea of a planet or large object permanently concealed by the Sun has long been a feature of science fiction. This concept centers on the Sun-Earth Lagrange Point 3 (L3), one of five points in space where gravitational forces and centripetal force are balanced. L3 is located precisely on the opposite side of the Sun from Earth, sharing our orbital path.
An object placed at L3 would theoretically remain hidden from Earth, but this point is considered unstable. The gravitational influence of other planets, particularly Venus, provides repeated nudges that would push any object away from the precise L3 location over time. Furthermore, observations have confirmed that no large, hidden “Counter-Earth” planet exists there. Advanced space-based instruments monitoring the region have detected no object larger than a few kilometers in diameter.
Satellite Monitoring of the Far Side
Scientists have overcome the line-of-sight obstruction through dedicated spacecraft and indirect observation methods. The most direct approach involves placing observation platforms into orbits significantly separated from Earth’s position. The Solar Terrestrial Relations Observatory (STEREO) mission, launched in 2006, used two identical spacecraft, one positioned ahead of Earth and one behind.
As the spacecraft drifted apart from Earth, they provided a nearly 360-degree view of the Sun’s surface by 2011. This configuration allowed researchers to observe solar phenomena on the far side before they rotated into Earth’s view. More recently, the European Space Agency’s Solar Orbiter mission has confirmed the ability to track solar activity from various vantage points, providing a more complete picture of the Sun’s global dynamics.
A secondary technique is helioseismology, which uses sound waves to map the Sun’s interior and far side. The Sun’s turbulent surface generates acoustic waves that travel through its interior, reflect off the far side, and return to the side facing Earth. Active regions with strong magnetic fields, like sunspots, affect the speed of these waves. By precisely measuring the travel time and Doppler shifts, scientists can create low-resolution images of magnetic activity on the far side. This non-visual method provides crucial data even when a spacecraft is not positioned to see the far side directly.
Solar Activity Beyond Our Sight
The far side of the Sun exhibits the same physical features and activities as the near side we observe from Earth. It is a dynamic landscape of sunspots, magnetic flux loops, and regions capable of producing solar flares and Coronal Mass Ejections (CMEs). CMEs are massive bursts of plasma and magnetic field that can be hurled into space.
The primary reason for monitoring the far side is space weather prediction. An active region that develops there can rotate to face Earth in less than two weeks, potentially launching a CME directly toward our planet. By tracking the size and complexity of sunspots and magnetic fields on the far side, scientists gain a lead time of several days to a week to forecast the severity of space weather impacts. Observations have tracked active regions, such as NOAA 13664 in 2024, from their emergence on the far side through multiple solar rotations.