Altair is the southern corner of the Summer Triangle; Vega and Deneb mark the other two corners. Look for the brightest star between two dimmer companions, Tarazed (Gamma Aquilae) and Alshain (Beta Aquilae). Together they make a short, nearly straight line sometimes called the Shaft of Aquila. The pattern is an excellent check on your identification, even where the larger Eagle is difficult to trace. Altair is about 16.7 light-years away, close enough to rank among our more familiar stellar neighbours. Sources: NASA Night Sky Network, https://science.nasa.gov/solar-system/skywatching/night-sky-network/summer-triangle-corner-altair/; *BBC Sky at Night*, https://www.skyatnightmagazine.com/advice/altair.
The star is a hot, hydrogen-burning member of the main sequence, but it does not rotate like the Sun. A spinning sphere of gas is pulled inward by gravity while its rotation reduces the effective inward pull most strongly at the equator. At sufficiently high speed, the equator bulges and the poles flatten. Astronomers call this shape *oblate*. A drawing of a perfect stellar sphere is therefore a useful simplification, not a universal law.
In 1999, the Palomar Testbed Interferometer measured Altair's apparent diameter along different directions. The result changed with the angle: the star was not circular. The researchers reported that its equatorial diameter was at least 14% greater than its polar diameter, and that it completed a rotation in no more than about 10.4 hours. Those were limits inferred from the observations then available, not exact modern measurements. Source: NASA Jet Propulsion Laboratory, December 1, 1999, https://www.jpl.nasa.gov/images/pia04204-altair/.
How can a telescope measure a star's outline without displaying a familiar photograph? *Interferometry* combines light collected at separated telescopes. Differences in the combined signals carry information about exceedingly small angular features. Palomar used pairs of small telescopes as a much longer effective baseline. The experiment was a measurement of shape rather than an ordinary close-up portrait; comparing Altair with Vega helped rule out an instrumental illusion.
A second consequence of Altair's spin is stranger still. Its equator is not only farther from the centre; it is cooler and less bright than its poles. The local effective gravity differs across the surface, affecting the temperature at which energy escapes. This is *gravity darkening*: the equator is relatively dim, not actually dark. In 2007, John Monnier and colleagues used the CHARA Array to reconstruct a near-infrared image of Altair. They saw the distorted surface and the brighter polar region. Their observations also showed that simple uniformly rotating models could not explain every detail; more complex rotation or temperature laws might be needed. Source: Monnier et al., *Science* 317 (2007), 342–345, https://doi.org/10.1126/science.1143205.
Later modelling sharpened the numbers. A 2020 two-dimensional study estimated an equatorial radius of 2.008 ± 0.006 solar radii, an inclination of 50.7 ± 1.2 degrees to our line of sight, and a rotation period of about 7 hours 46 minutes. The same model yielded a flattening of about 0.220. These are model-dependent estimates with stated uncertainties; they should not be confused with the older Palomar limits. Source: *Astronomy & Astrophysics*, “A realistic two-dimensional model of Altair,” https://doi.org/10.1051/0004-6361/201936830.
Long before those measurements, people gave the star other kinds of meaning. In the East Asian Cowherd and Weaver Girl tradition, Altair is associated with the cowherd and Vega with the weaver, separated in the sky by the Milky Way. Around the Mediterranean, Altair belonged to an eagle; NASA also records an eagle interpretation among Koori peoples of Australia. These are distinct traditions, not interchangeable versions of one story. Source: NASA Night Sky Network, https://science.nasa.gov/solar-system/skywatching/night-sky-network/summer-triangle-corner-altair/.
To observe Altair, begin with the Summer Triangle, then confirm the Tarazed–Altair–Alshain line. Binoculars can make the neighbouring stars easier to see, but no backyard telescope will reveal the flattened surface. At latitude 45° N, Altair culminates roughly 54° above an unobstructed southern horizon, calculated from its declination of about +8.9°. Check the Almanack's current-week chart for the best observing time and nearby planets. What you will see is one brilliant point; what astronomers have measured is a star whose rotation reshapes both its body and its light.
