ā, ī, ū, ŕ, ĺ , ai, au ( Aa ¸ [- ¸ } ¸ ?R ¸ ;R ¸ eo ¸ AaO ) are long.
â3, î3, û3, r3, l3, e3, ô3, ai3, au3 (Aa| ¸ [-| ¸ }| ¸ ?R| ¸ ;R| ¸ e| ¸ Aao| ¸ eo| ¸ AaO|) are protacted
The consonants combined with these metered vowels give a rhythmic, chanting pattern to the mantrās and these rhythms have seven basic structures called chhandâs . Now, interestingly the basic count for the number of syllables in these chhandâs varies from 24 to 48 increasing in steps of 4. These are gāyatrī (24), usnik (28), anustup (32), brhatī (36), pankti (40), tristup (44) and jagatī (48). Pt. Motilal Shastri (1908 – 1960), a vedic scholar of rare insight has written vigyānbhāsyam or scientific treatise on a number of Upaniśads and the Śatpath Brāhmana. He explains  that the Earth has a 24o ( 23.5o to be precise) tilt in its orbit, to us therefore the Sun appears to move from above the tropic of Capricorn to above the tropic of Cancer by an angle of 48o approx. in its annual swing. It is this degree of change that the Vedic chhandas reflect and this is further underlined by the fact that the middle chhanda is called brhatī (36), ‘the largest’ even though it is in the centre. This is because the Sun is the brightest at the Equator, in-between the two tropics.