Usum

Usum téh salah sahiji babagian utama taun, biasana mah dumasar bentuk iklim nu lega. Ilaharna sataun kabagi jadi opat usum, nyaéta: usum semi, usum panas, usum gugur, jeung usum tiris. Ngan, di Indonésia mah lantaran nganjrek di daérah tropis, mangka ngan dibagi jadi dua usum wungkul, nyaéta: usum ngijih jeung usum halodo.

Ari baheula di Mesir Kuna mah usum téh kabagi jadi tilu usum, nyaéta: musim caah, usum darurat lantaran caah, jeung usum panunéian nu panas.

Pananggalan	Beulahan bumi
	Tropis
April nepi ka Séptémber	usum halodo
Oktober nepi ka Pébruari	usum ngijih
	Lebahan kalér	Lebahan kidul
21 Maret-21 Juni	usum semi	usum gugur
21 Juni-23 Séptémber	usum panas	usum tiris
23 Séptémber-21 Désémber	usum gugur	usum semi
21 Désémber-21 Maret	usum tiris	usum panas

Usum dunya

Animasi pabédaan usum utamana salju panutup dina sataun

Tutumbu luar

• (Basa Inggris) The seasons begin at the time of the solstice or equinox
• (Basa Inggris) Solstice does not signal season's start?
• Australian Weather and Seasons Archived 2010-04-16 di Wayback Machine
• When do the Seasons Begin? (from the Bad Astronomer)
• Solstice does not signal season's start (from The Straight Dope)
• Why the Earth has seasons article on h2g2.
• Aboriginal seasons of Kakadu
• Indigenous seasons (Australian Bureau of Meteorology)
• Mt Stirling Seasons Archived 2012-03-02 di Wayback Machine
• Musim info Archived 2013-02-27 di Wayback Machine
• The Lost Seasons
• Melbourne's six seasons Archived 2009-09-19 di Wayback Machine
• The Lengths of the Seasons (numerical integration analysis)
• Earth distance vs tilt
• Tutorial on Earth/Sun Relations and Seasons Archived 2010-03-16 di Wayback Machine