Give the unimaginable
a useful scale.

This is the Schwarzschild model: an isolated, nonrotating, uncharged black hole. Its horizon radius is the areal radius, defined by a sphere’s area of 4πr². Real astrophysical black holes can rotate, changing the geometry.

Mass is entered in nominal solar units, using GM_☉ = 1.3271244 × 10²⁰ m³/s². The named mass presets are rounded comparisons, with observational uncertainty. Extra calculator digits describe the arithmetic, not extra measurement precision.

The horizon is a causal boundary. These lengths do not describe the accretion disk, the observed shadow, a material surface, or how long a spacecraft survives. The selectable mass range is a tool limit, not a list of confirmed black holes.

The estimate uses t = d/c, a straight path through vacuum, constant separation, and an immediate reply. It omits processing time, transmission duration, scheduling, blocked lines of sight, motion during the exchange, plasma effects, and relativistic corrections. Mission navigation uses more complete models.

Light travels at about 186,282.397 miles/s. The underlying SI value, c = 299,792.458 km/s, is exact. One AU is about 92.956 million miles, exactly 149,597,870.7 km. Miles here use 1 mile = 1.609344 km exactly. A light-year here is the distance light travels in a Julian year of 365.25 days, or 31,557,600 seconds. Extra display digits describe the arithmetic; they do not increase the precision of a rounded destination distance.

The Moon example uses a rounded mean center-to-center Earth–Moon distance. Sun–Earth L2 uses about 932,000 miles (1.5 million km) from Earth, yielding about 5 seconds one way and 10 seconds for an immediate reply. This is a location reference for Roman’s quasi-halo orbit, not the telescope’s live range or a distance from the Sun. For planets and Pluto, we put Earth on a 1 AU circle and the destination on a circle of radius a, then compare |a − 1| AU on the same side of the Sun with a + 1 AU on opposite sides. This geometry is our teaching inference from rounded orbital sizes, not an ephemeris. In particular, Pluto’s eccentric orbit makes its circle a coarse comparison. The Sun blocks the direct path at exact opposite-side alignment. An orbital distance from the Sun does not by itself equal a spacecraft’s distance from Earth.

The Oort Cloud uses a chosen 50,000 AU sample in an inferred, extended region. Proxima Centauri and Proxima Centauri b share the same rounded 4.24-light-year distance. Sagittarius A* uses 26,000 light-years to a hypothetical receiver outside the horizon; an inside-horizon signal cannot return. Andromeda uses about 2.5 million light-years. At such large distances, motion during the exchange is substantial enough that the simple doubled time should be read only as a static comparison. The 10-million-light-year input ceiling is a tool limit. This calculator does not model cosmological light travel or demonstrate that a signal would be detectable.

The starting dimensions are a 20 µm eukaryotic cell diameter, a 2 µm mitochondrial length, and a 0.1 nm atomic reference extent. Cells vary widely by type; mitochondria can change shape and form connected networks. Atoms have electron distributions whose extent depends on the atom, bonding and definition of size.

The drawings compare one linear dimension at a time. The mitochondrial width and internal folds are invented for the illustration. The atomic glow has no measured probability interpretation. These panels are scale diagrams, with no microscope magnification, molecular counting or resolution simulation.

The slider is logarithmic: one whole-number change in its underlying exponent changes the view width by a factor of ten. The ratios stay fixed while zoom changes. Only changing the chosen dimensions changes the ratios.