October 21

Joseph Aspdin, a Leeds bricklayer and builder, secured a patent for Portland cement on October 21, 1824. By burning a mixture of limestone and clay to produce a fine powder, Aspdin created a hydraulic binder that set stronger than previous lime mortars. He named it 'Portland cement' because its color resembled the famous Portland stone quarried in Dorset, England. This invention enabled the rapid expansion of infrastructure, bridges, and industrial buildings during the 19th century. Portland cement became the primary ingredient in concrete, transforming architectural and engineering possibilities. Its durability and versatility laid the foundation for modern skyscrapers, roads, and dams around the world. Today, Portland cement remains the most widely used construction material globally.

On October 21, 1879, Thomas Edison submits a patent application for an improved incandescent lamp capable of prolonged illumination. Building on previous experiments, Edison refines the filament and vacuum environment within the bulb. His design promises a practical and affordable source of electric light for homes and businesses. The patent is a critical milestone in Edison's pursuit of commercial electric lighting systems. It ultimately leads to the establishment of power stations and the widespread adoption of electric illumination worldwide.

On October 21, 1959, President Dwight D. Eisenhower authorized the transfer of all US Army space-related activities to the newly formed NASA. This historic decision included most assets of the Army Ballistic Missile Agency, headed by Wernher von Braun. The realignment aimed to centralize American civilian space exploration under a single agency. By absorbing Army research and launch facilities, NASA gained critical expertise for future missions. The transfer paved the way for the Mercury and Apollo programs that would define the Space Race. Eisenhower's move underscored the shift from military-driven rocketry to peaceful scientific exploration.

On October 21, 1965, Comet Ikeya–Seki reached perihelion, passing just 450,000 kilometers from the Sun's surface. The proximity made it one of the brightest comets of the 20th century, visible even in daylight. Discovered earlier that year by Japanese astronomers Kaoru Ikeya and Tsutomu Seki, it captivated skywatchers globally. Observers reported a brilliant coma and tail stretching across the sky, earning it nicknames like 'Comet of the Century.' Scientific teams used the event to study solar wind interactions and cometary composition. Its dramatic appearance remains a milestone in modern comet observations.

In October 1983, the 17th General Conference on Weights and Measures redefined the metre as the distance light travels in a vacuum in 1/299,792,458 of a second. This change replaced the artifact-based international prototype metre with a definition rooted in a fundamental natural constant. Scientists embraced the new standard for its precision and reproducibility, eliminating reliance on physical objects. The redefinition enhanced accuracy in fields ranging from geodesy to quantum physics. It underscored the scientific community's move toward SI units based on immutable properties of nature. Today, this definition remains the foundation for length measurements worldwide.

On October 21, 2005, telescopes provided the first visual confirmation of Eris, a distant object in the scattered disc region of the outer solar system. The images allowed astronomers to determine Eris’s orbit and estimate its mass, suggesting it was more massive than Pluto. This finding prompted the International Astronomical Union to reconsider the definition of a planet. In 2006, Eris was officially designated a dwarf planet alongside Pluto and other similar bodies. The discovery expanded our understanding of the solar system’s diversity and composition. Advanced imaging techniques used in capturing Eris underscored progress in observational astronomy during the early 21st century.