Ophthalmology

Pandemic learning can strain children's eyes

(HealthDay)—If your child will be doing online learning this school year, you need to take steps to protect them from eye strain, the American Academy of Ophthalmology says.

Diseases, Conditions, Syndromes

Viruses: ancient, tiny, amazing

They're as old as life itself but scientists can't say for sure if they're alive. They're written into our DNA, shaping the human saga through mutation and resilience.

Light-year

A light-year or light year (symbol: ly) is a unit of length, equal to just under 1013 kilometres. As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year.

The light-year is often used to measure distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications. The preferred unit in astrometry is the parsec, because it can be more easily derived from, and compared with, observational data. The parsec is defined as the distance at which an object will appear to move one arcsecond of parallax when the observer moves one astronomical unit perpendicular to the line of sight to the observer, and is equal to approximately 3.26 light-years.

The related unit of the light-month, roughly one-twelfth of a light-year, is also used occasionally for approximate measures.

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