Kungani sinezinkathi zonyaka, futhi ziyini?
Njengoba umhlaba uzungeza i - axis yawo, okwenza kube nobusuku nemini, uhamba futhi uzungeze ilanga emjikelezweni we - elliptical (isiyingi eside) odinga cishe izinsuku ezingu - 365 1/4. I - axis yomhlaba ejikelezayo ithambekele ohlangothini oluphambene nendawo yayo. Yilokhu okubangela izinkathi zonyaka.  Kunezinkathi ezine eziyinhloko zonyaka: Intwasahlobo, ihlobo, ikwindla nobusika. Izinsuku eziqala ngazo izinkathi zonyaka ngokwazo ziyahlukahluka kuye ngokuthi ukuyiphi ingxenye yomhlaba. Intwasahlobo nehlobo yizinkathi ezifudumele. Ngokuvamile ihlobo lihle, kanti ubusika yisikhathi esibanda kunazo zonke onyakeni.